Distributed feedback semiconductor laser and method for producing the same

ABSTRACT

In a distributed feedback semiconductor laser includes an InP substrate and a multiple layer structure formed on a main surface of the InP substrate, the multiple layer structure includes at least an active layer for emitting laser light and a periodical structure for distributed feedback of the laser light, and the periodical structure includes a plurality of semiconductor regions each having a triangular cross section in a direction perpendicular to the main surface of the InP substrate and parallel to a cavity length of the distributed feedback semiconductor laser, the triangular cross section projecting toward the InP substrate.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a distributed feedback semiconductorlaser suitable as a light source for long distance and large capacityoptical data communication, and a method for producing the same.

2. Description of the Related Art

Recently, distributed feedback semiconductor lasers (hereinafter,referred to as "DFB lasers") have been put into practical use as lightsources for long distance and large capacity optical data communicationand multi-channel video data transmission such as CATV. The DFB lasersemit light having a single wavelength and thus have such advantages ashigh response speed and low noise. As a result of such advantages, theDFB lasers are in wide use as light sources for optical datacommunication. Two methods for causing distributed feedback of lighthave been theoretically shown by, for example, Kogelnik ("Coupled-WaveTheory of Distributed Feedback Lasers", Journal of Applied Physics, vol.43, page 2327, 1972).

One of the methods is a refractive index coupled system, by which thesemiconductor laser is structured so that the refractive indexperiodically changes in the cavity length direction, and generates laseroscillation having a wavelength corresponding to the period of change(Bragg wavelength and the vicinity thereof). A great number of DFBlasers produced by this method have been reported because of ease inperforming the method. However, in a DFB laser produced by this method,laser oscillation theoretically occurs in either one of the two laseroscillation modes which interpose the Bragg wavelength and thus involvesa high possibility that laser oscillation occurs in both of the twooscillation modes.

The other method is a gain coupled system, by which the semiconductorlaser is structured so that the gain periodically changes in the cavitylength direction, and generates laser oscillation having a wavelengthcorresponding to the period of change (Bragg wavelength). In a DFB laserproduced in this method, oscillation theoretically occurs only at theBragg wavelength and thus has a high possibility that laser oscillationhas a single wavelength. However, a DFB laser having satisfactorycharacteristics has not been produced since the advent of the theorybecause of difficulties associated with performing the method.

Recently, a method for producing a DFB laser has been proposed wherebythe gain is periodically changed by providing an absorption layerperiodically in the semiconductor laser to generate satisfactory laseroscillation ("Long-Wavelength InGaAsP/InP Distributed Feedback LasersIncorporating Gain-Coupled Mechanism", Photonics Technology Letters,vol. 4, page 212, 1992). FIG. 18 is a cross sectional view of a DFBlaser having such a structure. N-type InGaAsP absorption layers 23 areburied periodically between n-type InP layers 21, 24 and 25, and ann-type InGaAsP optical waveguide layer 26, an active layer 29, a p-typeInGaAsP optical waveguide layer 30, and a p-type InP cladding layer 31.A bandgap energy of the n-type InGaAsP absorption layer 23 is set to besmaller than the emission energy from the active layer 29. Accordingly,the n-type InGaAsP optical waveguide layer 26 absorbs emission from theactive layer 29 periodically to cause a periodical change in the gain.Thus, there is a high possibility that a laser oscillation having asingle wavelength is produced.

A method for producing the DFB laser shown in FIG. 18 will be describedwith reference to FIGS. 19A, 19B and 19C.

As is shown in FIG. 19A, the n-type InGaAsP absorption layer 23 and ann-type InP passivation layer 24 are grown sequentially on an n-type InPsubstrate 21 by a first step of crystal growth. Then, as is shown inFIG. 19B, prescribed areas of the n-type InGaAsP absorption layer 23 areetched to form a diffraction grating 22 having a plurality of areas ofthe n-type InGaAsP absorption layers 23 arranged periodically. Then, asis shown in FIG. 19C, an n-type InP cladding layer 25 is deposited by asecond step of crystal growth to bury the n-type InGaAsP absorptionlayer 23.

This method requires etching the n-type InGaAsP absorption layer 23once. A surface of the n-type InGaAsP absorption layer 23 exposed byetching is subjected to heating by the second step of crystal growth.Because of such heating, the exposed surface can be undesirably defectedwhich degrades not only the optical characteristics of the n-typeInGaAsP absorption layer 23, but also the reliability of the DFB laserregarding the performance over a long period of time.

The "wavelength chirp", which is generated by direct modulation of thesemiconductor laser is a serious problem associated with long-distanceand large capacity data communication. For further improvement in thedata transmission characteristics, a light source having a lower levelof wavelength chirp is desired.

SUMMARY OF THE INVENTION

In one aspect of the present invention, in a distributed feedbacksemiconductor laser including an InP substrate and a multiple layerstructure formed on a main surface of the InP substrate, the multiplelayer structure includes at least an active layer for emitting laserlight and a periodical structure for distributed feedback of the laserlight, and the periodical structure includes a plurality ofsemiconductor regions each having a triangular cross section in adirection perpendicular to the main surface of the InP substrate andparallel to a cavity length of the distributed feedback semiconductorlaser, the triangular cross section projecting toward the InP substrate.

In one embodiment of the invention, the periodical structure is providedbetween the InP substrate and the active layer.

In one embodiment of the invention, the semiconductor regions are set tohave a bandgap energy which is smaller than the energy of light emittedfrom the active layer.

In one embodiment of the invention, the semiconductor regions are set tohave a bandgap energy which is at least equal to the energy of lightemitted from the active layer.

In one embodiment of the invention, the semiconductor regions areprovided in an InP cladding layer.

In one embodiment of the invention, the semiconductor regions areprovided between an InP cladding layer and an InGaAsP optical waveguidelayer.

In one embodiment of the invention, the active layer has a quantum wellstructure including at least a well layer and a barrier layer.

In one embodiment of the invention, a compression strain is induced intothe well layer.

In one embodiment of the invention, the compression strain is induced at0.5 to 1.5%.

In one embodiment of the invention, the semiconductor regions are formedof InAsP.

In one embodiment of the invention, the semiconductor regions are formedat least of a first semiconductor material having a bandgap energy whichis smaller than the energy of light emitted from the active layer and asecond semiconductor material having a bandgap energy which is largerthan the energy of the light.

In one embodiment of the invention, the semiconductor regions are formedat least of a first semiconductor material having a bandgap energy whichis smaller than the energy of light emitted from the active layer and asecond semiconductor material having a bandgap energy which is largerthan the energy of the light.

In one embodiment of the invention, the semiconductor regions have anaverage refractive index which is substantially equal to the refractiveindex of InP.

In one embodiment of the invention, the first semiconductor material isInAsP, and the second semiconductor material is InGaP.

In another aspect of the present invention, a method for producing adistributed feedback semiconductor laser includes the steps ofperiodically forming a plurality of grooves at a surface of an InPsubstrate; heating the InP substrate in an atmosphere containing atleast a mixture of phosphine and arsine to grow an InAsP layer in eachof the grooves; and forming a multiple layer structure including anactive layer on the InP substrate, covering the InAsP layers.

In still another aspect of the present invention, a method for producinga distributed feedback semiconductor laser includes the steps of forminga multiple layer structure on an InP substrate, the multiple layerincluding an active layer and an InP top layer; periodically forming aplurality of grooves at a surface of the InP top layer; and heating themultiple layer structure in an atmosphere containing at least a mixtureof phosphine and arsine to grow an InAsP layer in each of the grooves.

In one embodiment of the invention, the method further includes the stepof growing an InGaP layer on the InAsP layers before the formation ofthe multiple layer structure.

In one embodiment of the invention, the method further includes the stepof growing an InGaP layer on the InAsP layers.

In still another aspect of the present invention, in a distributedfeedback semiconductor laser including an InP substrate and a multiplelayer structure formed on a main surface of the InP substrate, themultiple layer structure includes at least an active layer for emittinglaser light and a periodical structure for distributed feedback of thelaser light, and the periodical structure includes a plurality ofsemiconductor regions each having a triangular cross section in adirection perpendicular to the main surface of the InP substrate andparallel to a cavity length of the distributed feedback semiconductorlaser, the triangular cross section projecting toward the InP substrate.

In one embodiment of the invention, the semiconductor regions are formedat least of a first semiconductor material having a bandgap energy whichis smaller than the energy of light emitted from the active layer and asecond semiconductor material having a bandgap energy which is largerthan the energy of the light.

In one embodiment of the invention, the semiconductor regions areprovided between the InP substrate and the active layer.

In still another aspect of the present invention, a crystal growthmethod includes corrugating a surface of a layer formed of InP crystalsby etching; and heating the InP crystals in an atmosphere including atleast a mixture of phosphine and arsine to grow an InAsP layer ingrooves of the corrugated surface.

In still another aspect of the present invention, in a distributedfeedback semiconductor laser including an InP substrate and a multiplelayer structure formed on a main surface of the InP substrate, themultiple layer structure includes at least an active layer for emittinglaser light and a periodical structure for treating the laser light withdistributed feedback, the periodical structure includes a plurality offirst semiconductor regions and a plurality of second semiconductorregions arranged alternately in a direction of a cavity length of thedistributed feedback semiconductor laser, and the first semiconductorregions each have a triangular cross section in a directionperpendicular to the main surface of the InP substrate and parallel tothe cavity length, the triangular cross section projecting toward theInP substrate.

In one embodiment of the invention, the first semiconductor regions areset to have a bandgap energy which is smaller than the energy of lightemitted from the active layer and the second semiconductor regions areset to have a bandgap energy which is larger than the energy of thelight.

In one embodiment of the invention, the first and the secondsemiconductor regions are formed in an InP cladding layer.

In one embodiment of the invention, the first and the secondsemiconductor regions are formed between an InP cladding layer and anInGaAsP optical waveguide layer.

In one embodiment of the invention, the first semiconductor regions areformed of InAsP, and the second semiconductor regions are formed ofInGaP.

In still another aspect of the present invention, a distributed feedbacksemiconductor laser includes a striped multiple layer structureincluding a light emission part for emitting laser light and amodulation part which is optically coupled with the emission part formodulating the laser light; and a semiconductor substrate for supportingthe striped multiple layer structure. The emission part includes a gaincoupled cavity having an active layer and an absorption type diffractiongrating which absorbs light from the active layer at an absorbanceperiodically changing in a direction of an optical axis, and Themodulation part includes a light modulation layer having opticalcharacteristics which changes in accordance with a modulation signal.

In one embodiment of the invention, the striped multiple layer structureincludes a multiple quantum well layer, the active layer in the emissionpart includes a first part of the multiple quantum well layer, and thelight modulation layer in the modulation part includes a second part ofthe multiple quantum well layer.

In one embodiment of the invention, the active layer in the emissionpart and the light modulation layer in the modulation part are connectedto each other by a third part of the multiple quantum well layer locatedbetween the first part and the second part.

In one embodiment of the invention, the first part of the multiplequantum well layer is thicker than the second part of the multiplequantum well layer.

In one embodiment of the invention, the absorption type diffractiongrating includes a plurality of light absorption layers arranged in thedirection of the optical axis.

In one embodiment of the invention, the diffraction grating includes aplurality of light absorption layers which have a thickness changingperiodically in the direction of the optical axis and have a bandgapchanging periodically in the direction of the optical axis in accordancewith the periodical change in the thickness.

In one embodiment of the invention, the light absorption layers have aquantum well structure.

In one embodiment of the invention, the light absorption layers aregrown on periodical corrugations formed at a surface of thesemiconductor substrate.

In one embodiment of the invention, the absorption type diffractiongrating is provided between the active layer and the semiconductorsubstrate.

In one embodiment of the invention, the active layer is provided betweenthe absorption type diffraction grating and the semiconductor substrate.

In one embodiment of the invention, the distributed feedbacksemiconductor laser further includes an optical waveguide layer providedbetween the active layer and the absorption type diffraction grating.

In one embodiment of the invention, the distributed feedbacksemiconductor laser further includes a first voltage application devicefor applying a substantially constant voltage to the emission part and asecond voltage application device for applying a modulation voltage tothe modulation part.

In one embodiment of the invention, the light absorption layers areformed of InAsP.

In still another aspect of the present invention, a distributed feedbacksemiconductor laser includes a semiconductor substrate having a firstcladding layer of a first conductivity type having a bandgap λg1; and astriped multiple layer structure including a first part and a secondpart which are optically coupled to each other due to continuity of thestriped multiple layer structure along an identical optical axis. Thefirst part includes a cavity structure including an active layer havinga bandgap wavelength λg2, an optical waveguide layer of the firstconductivity type having a bandgap wavelength λg3, a second claddinglayer of a second conductivity type having a bandgap wavelength λg1, anda light absorption layer which has a bandgap wavelength λg4, and isburied between the optical waveguide layer and the semiconductorsubstrate to form an absorption type diffraction grating periodical in adirection of an optical axis. The second part includes a lightmodulation layer having a bandgap wavelength λg5, and a third claddinglayer of the second conductivity type having a bandgap wavelength λg1,both of which are grown in a different growth step from the activelayer, the optical waveguide layer, the second cladding layer, and thelight absorption layer in the first part. The bandgap wavelengths havethe relationship of λg4>λg2>λg3>λg1 and λg2>λg5>λg1. A Bragg wavelengthλ_(B) determined by an effective refractive index of the cavity in thefirst part and the periodicity of the absorption type diffractiongrating is set in a range including λg2.

In still another aspect of the present invention, a distributed feedbacksemiconductor laser includes a semiconductor substrate having a firstcladding layer of a first conductivity type having a bandgap λg1; and astriped multiple layer structure including a first part and a secondpart which are optically coupled to each other due to continuity of thestriped multiple layer structure along an identical optical axis. Thefirst part includes a cavity structure including a multiple quantum wellactive layer having a bandgap wavelength λg2, a first optical waveguidelayer of the first conductivity type having a bandgap wavelength λg3, asecond cladding layer of a second conductivity type having a bandgapwavelength λg1, and a quantum well light absorption layer which has abandgap wavelength λg4 and is buried between the first optical waveguidelayer and the semiconductor substrate to form an absorption typediffraction grating periodical in a direction of the optical axis. Thesecond part includes a buried quantum well layer having a bandgapwavelength λg6, a second optical waveguide layer having a bandgapwavelength λg2, a multiple quantum well light modulation layer having abandgap wavelength λg5, and a third cladding layer of the secondconductivity type having a bandgap wavelength λg1. The quantum welllight absorption layer in the first part and the buried quantum welllayer in the second part are formed in the same growth stepsimultaneously, the first optical waveguide layer in the first part andthe second optical waveguide layer in the second part are formed in thesame growth step simultaneously, the multiple quantum well active layerin the first part and the multiple quantum well light modulation layerin the second part are formed in the same growth step simultaneously,and the second cladding layer in the first part and the third claddinglayer in the second part are formed in the same growth stepsimultaneously. The bandgap wavelengths have the relationship ofλg4>λg2>λg3>λg1, λg6<λg4, and λg5<λg2. A Bragg wavelength λ_(B)determined by an effective refractive index of the cavity in the firstpart and the periodicity of the absorption type diffraction grating isset in a range including λg2.

In still another aspect of the present invention, a distributed feedbacksemiconductor laser includes a semiconductor substrate having a firstcladding layer of a first conductivity type having a bandgap λg1; and astriped multiple layer structure including a first part and a secondpart which are optically coupled to each other due to continuity of thestriped multiple layer structure along an identical optical axis. Thefirst part includes a cavity structure including a multiple quantum wellactive layer having a bandgap wavelength λg2, a first optical waveguidelayer of the first conductivity type having a bandgap wavelength λg3, asecond cladding layer of a second conductivity type having a bandgapwavelength λg1, and a quantum well light absorption layer which isburied between the first optical waveguide layer and the first claddinglayer to form an absorption type diffraction grating. The quantum welllight absorption layer has a thickness which periodically changes thusto periodically change the bandgap wavelength thereof, and the thicknessand the composition of the quantum well light absorption layer are setso that the bandgap has a maximum value larger than λg2 and a minimumvalue smaller than λg2. The second part includes a quantum well layerhaving a bandgap wavelength λg5, a second optical waveguide layer havinga bandgap wavelength λg3, a multiple quantum well light modulation layerhaving a bandgap wavelength λg6, and a third cladding layer of thesecond conductivity type having a bandgap wavelength λg1. The buriedquantum well light absorption layer in the first part and the quantumwell layer in the second part are formed in the same growth stepsimultaneously, the first optical waveguide layer in the first part andthe second optical waveguide layer in the second part are formed in thesame growth step simultaneously, the multiple quantum well active layerin the first part and the multiple quantum well light modulation layerin the second part are formed in the same growth step simultaneously,and the second cladding layer in the first part and the third claddinglayer in the second part are formed in the same growth stepsimultaneously. The bandgap wavelengths have the relationship ofλg4>λg2>λg3≧λg1, λg5<λg2, and λg6<λg2. A Bragg wavelength λ_(B)determined by an effective refractive index of the cavity in the firstpart and the periodicity of the absorption type diffraction grating isset in a range including λg2.

In still another aspect of the present invention, a distributed feedbacksemiconductor laser includes a semiconductor substrate having a firstcladding layer of a first conductivity type having a bandgap λg1; and astriped multiple layer structure including a first part and a secondpart which are optically coupled to each other due to continuity of thestriped multiple layer structure along an identical optical axis. Thefirst part includes a cavity structure including an active layer havinga bandgap wavelength λg2, an optical waveguide layer of the firstconductivity type having a bandgap wavelength λg3, a second claddinglayer of a second conductivity type having a bandgap wavelength λg1, afirst spacer layer which has a bandgap wavelength of λg1 and is providedbetween the optical waveguide layer and the active layer, and a lightabsorption layer which has a bandgap wavelength λg4 and is buried in thesecond cladding layer to form an absorption type diffraction gratingperiodical in a direction of the optical axis. The second part includesa light modulation layer having a bandgap wavelength λg5, a thirdcladding layer of the second conductivity type having a bandgapwavelength λg1, and a second spacer layer which has a bandgap wavelengthof λg1 and is provided between the light modulation layer and the thirdcladding layer. The optical waveguide layer in the first part and thelight modulation layer in the second part are formed in the same growthstep simultaneously, and the first spacer layer in the first part andthe second spacer layer in the second part are formed in the same growthstep simultaneously. The bandgap wavelengths have the relationship ofλg4>λg2>λg3>λg1 and λg2>λg5>λg1. A Bragg wavelength λ_(B) determined byan effective refractive index of the cavity in the first part and theperiodicity of the absorption type diffraction grating is set in a rangeincluding λg2.

In still another aspect of the present invention, a semiconductor laserincludes: a striped multiple layer structure including a light emissionpart for emitting laser light and an optical waveguide part which isoptically coupled with the light emission part for propagating the laserlight therethrough; and a semiconductor substrate for supporting thestriped multiple layer structure. The light emission part includes anactive layer radiating the laser light, the waveguide part includes aBragg reflector where a refractive index periodically changes along anoptical axis direction, and the Bragg reflector includes an InAsP layerformed in a concave part of periodic corrugations formed on thesemiconductor substrate so as to reflect light having a selectedwavelength of the laser light radiated from the active layer of theemission part toward the active layer.

In one embodiment of the invention, the InAsP layer has a bandgap energynot to absorb the laser light.

In one embodiment of the invention, the optical waveguide part includesa wavelength tuning part for adjusting a wavelength of the laser light.

In one embodiment of the invention, the optical waveguide part includesa phase control part for adjusting a phase of the laser light.

Thus, the invention described herein makes possible the advantages ofproviding a DFB laser for causing laser oscillation having a singlewavelength, maintaining the oscillation mode against returning lightfrom outside the DFB laser, and withstanding noise; and a method forproducing the same.

These and other advantages of the present invention will become apparentto those skilled in the art upon reading and understanding the followingdetailed description with reference to the accompanying figures.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a partially cut isometric view of a DFB laser in a firstexample according to the present invention;

FIGS. 2A through 2E are isometric views illustrating a method forproducing the DFB laser shown in FIG. 1;

FIG. 3A through 3C are cross sectional views illustrating a specificpart of the method for producing the DFB laser shown in FIG. 1;

FIG. 4 is a graph illustrating a photoluminescence spectrum obtainedfrom an InAsP layer formed in the method shown in FIGS. 2A through 2E;

FIG. 5 is a graph illustrating how to control the bandgap energy of theInAsP layer;

FIG. 6 is a partial cross sectional view of the DFB laser shown in FIG.1 for describing a diffraction grating provided with InGaP refractiveindex compensation layers;

FIG. 7 is a graph illustrating the electric current vs. optical outputcharacteristic of the DFB laser shown in FIG. 1;

FIG. 8 is a graph illustrating a spectral characteristic of the DFBlaser shown in FIG. 1;

FIG. 9 is a partially cut isometric view of a DFB laser in a secondexample according to the present invention;

FIG. 9A is a partially cut isometric view of a DFB laser as shown inFIG. 9, but with the cladding layer replaced with an optical waveguidelayer.

FIGS. 10A through 10F are isometric views illustrating a method forproducing the DFB laser shown in FIG. 9;

FIG. 11 is a partially cut isometric view of a DFB laser in a thirdexample according to the present invention;

FIGS. 12A through 12F are isometric views illustrating a method forproducing the DFB laser shown in FIG. 11;

FIG. 13 is a partially cut isometric view of a DFB laser in a fourthexample according to the present invention;

FIGS. 14A through 14F are isometric views illustrating a method forproducing the DFB laser shown in FIG. 13;

FIG. 15 is a partially cut isometric view of a DFB laser in a fifthexample according to the present invention;

FIGS. 16A through 16F are isometric views illustrating a method forproducing the DFB laser shown in FIG. 15;

FIGS. 17A through 17D are cross sectional views of the DFB laser shownin FIG. 15 for describing a diffraction grating provided with InAsPrefractive index compensation layers;

FIG. 18 is a cross sectional view of a conventional DFB laser;

FIG. 19A through 19C are cross sectional views illustrating a method forproducing the conventional DFB laser shown in FIG. 18;

FIG. 20 is a cross sectional view of a DFB laser in a sixth example;

FIG. 21 is a cross sectional view of a DFB laser in a seventh example;

FIG. 22 is a cross sectional view of a DFB laser in an eighth example;

FIG. 23 is a cross sectional view of a DFB laser in a ninth example;

FIG. 24 is a cross sectional view of a DFB laser in a tenth example;

FIG. 25A is a cross sectional view of a semiconductor substrate havingcorrugations in the DFB laser in the tenth example;

FIG. 25B is a plan view of a striped mask used in the production in theDFB laser in the tenth example; and

FIG. 25C is a cross sectional view of the semiconductor substrateprovided with an InAsP layer in the DFB laser in the tenth example;

FIG. 26 is a cross sectional view of a DBR laser in an eleventh example;

FIG. 27 is a cross sectional view of a DBR laser in a twelfth example.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

Hereinafter, the present invention will be described by way ofillustrative examples with reference to the accompanying drawings.

EXAMPLE 1

FIG. 1 is a partially cut isometric view of a DFB laser 100 in a firstexample according to the present invention.

The DFB laser 100 has a mesa structure on a top main surface (topsurface) of an n-type InP substrate 1, the mesa structure including ann-type InP cladding layer 4 (thickness: 200 nm), an n-type InGaAsPoptical waveguide layer 5 (thickness: 50 nm; bandgap wavelength λg=1.05μm), a multiple quantum well active layer (hereinafter, referred to as a"MQW active layer") 6, a p-type InGaAsP optical waveguide layer 7(thickness: 30 nm; λg=1.05 μm), and a p-type InP cladding layer 15(thickness: 400 nm).

The mesa structure is provided on corrugations 2 formed at the topsurface of the n-type InP substrate 1. Laterally, the mesa structure isinterposed by a p-type InP current blocking layer 8 and an n-type InPcurrent blocking layer 9 formed on the n-type InP substrate 1. On theabove-mentioned semiconductor layers, a p-type InP burying layer 10 anda p-type InGaAsP contact layer 11 (λg=1.3 μm) are formed.

On a main surface of the n-type InP substrate 1 which does not have themesa structure thereon (bottom surface), an n-type electrode 14 formedof an Au/Sn alloy is provided. On a top surface of the p-type InGaAsPcontact layer 11, an insulation layer 12 formed of SiO₂ having astripe-shaped window is formed. A p-type electrode 13 provided tosubstantially cover the insulation layer 12 is in contact with thep-type InGaAsP contact layer 11 through the window.

Between the n-type InP substrate 1 and the n-type InP cladding layer 4,a plurality of InAsP absorption layers 3 are arranged in the cavitylength direction at a pitch of 203 nm. Each InAsP absorption layer 3 hasa triangular cross section parallel to the cavity length direction andperpendicular to the main surfaces of the n-type InP substrate 1. Anapex of the triangle projects into the n-type InP substrate 1. Since theInAsP absorption layer 3 has such a shape, the absorption layer is notrequired to be etched. It is very difficult to form an absorption layer,such as the absorption layer 3 of this example, with a small size on theorder of several ten nanometers by etching. Even when the absorptionlayer is formed by etching, the resulting size of the absorption layeris likely to be largely varied. Since the etching is not necessary, itis possible to easily and uniformly form an absorption layer with asmall size of about several ten nanometers, thereby considerablyreducing the variation of the characteristics of the respectiveelements.

The MQW active layer 6 includes ten quantum wells, each quantum wellincluding an InGaAsP well layer (thickness: 6 nm) and an InGaAsP barrierlayer (thickness: 10 nm; λg=1.05 μm). Strain is induced by compressioninto the InGaAsP well layer, and is not induced intentionally into theInGaAsP barrier layer.

The photoluminescence wavelength of the InAsP absorption layer 3periodically formed in the cavity length direction is set to be 1.4 μm,and the oscillation wavelength from the MQW active layer 6 is set to be1.3 μm. Due to such a structure, the InAsP absorption layer 3 causes aperiodical change in the gain in the cavity length direction. Thus,laser oscillation having a single wavelength is provided at a higherpossibility than the laser oscillation generated by the periodicalchange only in the refractive index.

A method for producing the DFB laser 100 will be described withreference to FIGS. 2A through 2E.

As is shown in FIG. 2A, the corrugations 2 having a pitch of 203 nm anda maximum depth of approximately 100 nm are formed at the top surface ofthe n-type InP substrate 1 by holographic exposure.

Next, a hydrogen atmosphere is mixed with 100% phosphine (PH₃) at a rate100 cc/min. and 10% arsine (AsH₃) at a rate of 10 cc/min. In theresultant atmosphere, the n-type InP substrate 1 is heated at 600° C. Asa result, as is shown in FIG. 2B, the InAsP absorption layers 3 having athickness of approximately 50 nm are formed in a plurality of grooves ofthe corrugations 2 to form a diffraction grating. Then, as is shown inFIG. 2C, the n-type InP cladding layer 4, the n-type InGaAsP opticalwaveguide layer 5, the MQW active layer 6 having the above-describedstructure, the p-type InGaAsP optical waveguide layer 7, the p-type InPcladding layer 15, and a p-type InGaAsP capping layer 16 (λg=1.3 μm) aresequentially grown on the InAsP absorption layers 3 by metal organicvapor phase epitaxy (hereinafter, referred to as "MOVPE").

Thereafter, as is shown in FIG. 2D, the mesa stripe is formed byetching. Then, the p-type InP current blocking layer 8, the n-type InPcurrent blocking layer 9, the p-type InP burying layer 10, and thep-type InGaAsP contact layer 11 are sequentially grown by liquid phaseepitaxy. The insulation layer 12 formed of SiO₂ is deposited on thep-type InGaAsP contact layer 11 and a stripe-shaped window is formed.Then, the p-type electrode 13 is formed by evaporation. On the bottomsurface of the n-type InP substrate 1, the n-type electrode 14 is formedby evaporation. The resultant body is cleaved to obtain the DFB laser100 as is shown in FIG. 2E.

By the above-described method in this example, the absorption layers 3are not formed by etching. Accordingly, even when the temperature israised for forming the above-mentioned semiconductor layers after themesa stripe is formed by etching, atoms are not evaporated from thesurfaces of the InAsP absorption layer 3.

The DFB laser 100 in this example can also be easily produced by simplygrowing the semiconductor layers after the formation of the corrugations2.

A particular step of the above-described method will be described withreference to FIGS. 3A through 3C.

FIG. 3A is a cross sectional view of the n-type InP substrate 1 havingthe corrugations 2 formed by etching. By heating the n-type InPsubstrate 1 with the corrugations 2 in a hydrogen atmosphere mixed withphosphine (PH₃) and arsine (AsH₃), the InAsP absorption layers 3 areformed in the grooves of the corrugations 2 by mass-transport duringheating, as is shown in FIG. 3B. By growing the n-type InP claddinglayer 4, the InAsP absorption layers 3 having an inverted triangularcross section and arranged periodically are obtained, as shown in FIG.3C. From the point of light absorbance, the thickness of each InAsPabsorption layer 3 is preferably 10 nm or more.

FIG. 4 illustrates the photoluminescence spectrum from the InAsPabsorption layer 3 produced in this manner at room temperature. Aspectrum having a single peak at around 1.5 μm is observed. In the caseof an InAsP absorption layer produced on the n-type InP substrate 1without forming a diffraction grating, no emission was observed exceptfor emission from the n-type InP substrate 1.

FIG. 5 illustrates the photoluminescence wavelength from the InAsPabsorption layer 3 with respect to the flow rate of arsine in the casewhere 100% phosphine is induced to the hydrogen atmosphere at a rate of100 cc/min at 600° C. As is appreciated from FIG. 5, when the flow rateof arsine is changed while the flow rate of phosphine is kept the same,the photoluminescence wavelength from the InAsP absorption layer 3changes continuously. Such a result indicates that the bandgap energy ofthe InAsP absorption layer 3 can be changed by changing the flow rate ofarsine.

When the bandgap energy is set to be larger than the energy of the lightemitted from an active layer after being treated with distributedfeedback, namely, when the photoluminescence wavelength from an InAsPabsorption layer is set to be shorter than the oscillation wavelength ofthe laser, the InAsP absorption layer allows the light emitted from theactive layer to transmit therethrough. Since the InAsP absorption layerhas a higher refractive index than InP in the vicinity thereof, therefractive index changes periodically. The DFB laser produced in thismanner is of the refractive index coupled type.

When the bandgap energy is set to be smaller than the energy of thelight emitted from an active layer after being treated with distributedfeedback, namely, when the photoluminescence wavelength from an InAsPabsorption layer is set to be longer than the oscillation wavelength ofthe laser, the InAsP absorption layer absorbs the light emitted from theactive layer. Accordingly, the gain changes periodically. The DFB laserproduced in this manner is of the gain coupled type. The DFB laser 100in this example is of the gain coupled type.

In the case where the periodical change in the gain is caused by growingthe InAsP absorption layers 3, the periodical change in the refractiveindex is also caused because the refractive index of the InAsPabsorption layer 3 is larger than that of InP. FIG. 6 is a crosssectional view of a structure including InGaP refractive indexcompensation layers 18. The InGaP refractive index compensation layers18 each have a smaller refractive index than InP and is grown after theformation of the InAsP absorption layers 3 in the grooves of thecorrugations 2. In such a structure, an average refractive index of theInAsP absorption layer 3 and the InGaP refractive index compensationlayer 18 can be adjusted by the thicknesses of these two layers 3 and18. In this manner, flexibility in designing the laser is expanded. Thiseffect will be described in more detail in Example 3.

FIG. 7 illustrates the optical output vs. current characteristic in theDFB laser 100. The characteristic is excellent with the thresholdcurrent of 16 mA and the slope efficiency of 0.25 mW/mA. FIG. 8illustrates the oscillation spectrum of the DFB laser 100. The side moderestriction ratio, which indicates the ratio of the main oscillationmode with respect to the side oscillation mode, is 40 dB or more, whichsignifies that stable laser oscillation having a single wavelength isobtained. Considering that the side mode restriction ratio of a generalDFB laser of the refractive index coupled type is approximately 30 dB,the DFB laser 100 has an advantage of being the gain coupled type.

The n-type InP cladding layer 4 can be replaced with an n-type InGaAsPwaveguide layer to be integral with the n-type InGaAsP optical waveguidelayer 5.

The DFB laser 100, which is of the gain coupled type, generates laseroscillation having a single wavelength and is stable against the lightreturned from outside the DFB laser 100.

EXAMPLE 2

FIG. 9 is a partially cut isometric view of a DFB laser 200 in a secondexample according to the present invention. Identical elements as thosein the first example will bear identical reference numerals therewithand a description thereof will be omitted.

The DFB laser 200 has a mesa structure on a top main surface of ann-type InP substrate 1, the mesa structure including an n-type InPcladding layer 4 (thickness: 200 nm), an n-type InGaAsP opticalwaveguide layer 5 (thickness: 50 nm; λg=1.05 μm), an MQW active layer 6,a p-type InGaAsP optical waveguide layer 7 (thickness: 30 nm; λg=1.05μm), a first p-type InP cladding layer 15 (average thickness: 100 nm),and a second p-type InP cladding layer 17 (average thickness: 300 nm).The mesa structure is provided on the top surface of the n-type InPsubstrate 1. Laterally, the mesa structure is interposed by a p-type InPcurrent blocking layer 8 and an n-type InP current blocking layer 9formed on the n-type InP substrate 1. On the above-mentionedsemiconductor layers, a p-type InP burying layer 10 and a p-type InGaAsPcontact layer 11 (λg=1.3 μm) are formed.

On a main surface of the n-type InP substrate 1 which does not have themesa structure thereon (bottom surface), an n-type electrode 14 formedof an Au/Sn alloy is provided. On a top surface of the p-type InGaAsPcontact layer 11, an insulation layer 12 formed of SiO₂ having astripe-shaped window is formed. A p-type electrode 13 provided tosubstantially cover the insulation layer 12 is in contact with thep-type InGaAsP contact layer 11 through the window.

Between the first p-type InP cladding layer 15 and the second p-type InPcladding layer 17, a plurality of InAsP absorption layers 3 are arrangedin the cavity length direction at a pitch of 203 nm. Each InAsPabsorption layer 3 has a triangular cross section parallel to the cavitylength direction and perpendicular to the main surfaces of the n-typeInP substrate 1, and an apex of the triangle projects into the n-typeInP substrate 1.

The MQW active layer 6 includes ten quantum wells, each quantum wellincluding an InGaAsP well layer (thickness: 6 nm) and an InGaAsP barrierlayer (thickness: 10 nm; λg=1.05 μm). Strain is induced by compressioninto the InGaAsP well layer, and is not induced intentionally into theInGaAsP barrier layer.

The photoluminescence wavelength of the InAsP absorption layer 3periodically formed in the cavity length direction is set to be 1.4 μm,and the oscillation wavelength from the MQW active layer 6 is set to be1.3 μm. Due to such a structure, the InAsP absorption layer 3 causes aperiodical change in the gain in the cavity length direction. Thus,laser oscillation having a single wavelength is provided at a higherpossibility than the laser oscillation generated by the periodicalchange only in the refractive index for the reasons described above.

In this example, the InAsP absorption layers 3 are formed above the MQWactive layer 6. In the structure of the first example, the n-type InPcladding layer 4 and the n-type InGaAsP optical waveguide layer 5interposed between the InAsP absorption layers 3 and the MQW activelayer 6 each preferably have a relatively large thickness in order torecover the crystallinity. In the structure in the second example, thethickness of each of the p-type InGaAsP optical waveguide layer 7 andthe first p-type InP cladding layer 15 interposed between the MQW activelayer 6 and the InAsP absorption layers 3 can be set relatively freely.This indicates that the optical intensity distribution and the degree ofcoupling due to the InAsP absorption layer 3 can be set more freely thanin the first example.

As shown in FIG. 9A, the first p-type InP cladding layer 15 of FIG. 9can be replaced with a p-type InGaAsP optical waveguide layer 7a to beintegral with the p-type InGaAsP optical waveguide layer 7.

A method for producing the DFB laser 200 will be described withreference to FIGS. 10A through 10F.

As is shown in FIG. 10A, the n-type InP cladding layer 4, the n-typeInGaAsP optical waveguide layer 5, the MQW active layer 6 having theabove-described structure, and the p-type InGaAsP optical waveguidelayer 7, the first p-type InP cladding layer 15 are sequentially grownon the n-type InP substrate 1 by MOVPE. Then, as is shown in FIG. 10B,corrugations 2 are formed having a pitch of 203 nm and a maximum depthof approximately 100 nm is formed by holographic exposure.

Next, a hydrogen atmosphere is mixed with 100% phosphine (PH₃) at a rate100 cc/min. and 10% arsine (AsH₃) at a rate of 10 cc/min. In theresultant atmosphere, the n-type InP substrate 1 is heated at 600° C. Asa result, as is shown in FIG. 10C, the InAsP absorption layers 3 havinga thickness of approximately 50 nm are formed in the grooves of thecorrugations 2 to form a diffraction grating. Then, as is shown in FIG.10D, the second p-type InP cladding layer 17 (thickness: 300 nm) and ap-type InGaAsP capping layer 16 (λg=1.3 μm) are sequentially formed onthe InAsP absorption layers 3.

Thereafter, as is shown in FIG. 10E, the mesa stripe is formed byetching. Then, the p-type InP current blocking layer 8, the n-type InPcurrent blocking layer 9, the p-type InP burying layer 10, and thep-type InGaAsP contact layer 11 are sequentially grown by liquid phaseepitaxy. The insulation layer 12 formed of SiO₂ is deposited on thep-type InGaAsP contact layer 11 and a stripe-shaped window is formed.Then, the p-type electrode 13 is formed by evaporation. On the bottomsurface of the n-type InP substrate 1, the n-type electrode 14 is formedby evaporation. The resultant body is cleaved to obtain the DFB laser200 as is shown in FIG. 10F.

EXAMPLE 3

FIG. 11 is a partially cut isometric view of a DFB laser 300 in a thirdexample according to the present invention. Identical elements as thosein the first example will bear identical reference numerals therewithand a description thereof will be omitted.

The DFB laser 300 has a mesa structure on a top main surface of ann-type InP substrate 1, the mesa structure including an n-type InPcladding layer 4 (thickness: 200 nm), an n-type InGaAsP opticalwaveguide layer 5 (thickness: 50 nm; λg=1.05 μm), an MQW active layer 6,a p-type InGaAsP optical waveguide layer 7 (thickness: 30 nm; λg=1.05μm), and a p-type InP cladding layer 15 (thickness: 400 nm). The mesastructure is provided on corrugations 2 formed at the top surface of then-type InP substrate 1. Laterally, the mesa structure is interposed by ap-type InP current blocking layer 8 and an n-type InP current blockinglayer 9 formed on the n-type InP substrate 1. On the above-mentionedsemiconductor layers, a p-type InP burying layer 10 and a p-type InGaAsPcontact layer 11 (λg=1.3 μm) are formed.

On a main surface of the n-type InP substrate 1 which does not have themesa structure thereon (bottom surface), an n-type electrode 14 formedof an Au/Sn alloy is provided. On a top surface of the p-type InGaAsPcontact layer 11, an insulation layer 12 formed of SiO₂ having astripe-shaped window is formed. A p-type electrode 13 provided tosubstantially cover the insulation layer 12 is in contact with thep-type InGaAsP contact layer 11 through the window.

Between the n-type InP substrate 1 and the n-type InP cladding layer 4,a plurality of InAsP absorption layers 3 and a plurality of InGaPrefractive index compensation layers 18 are arranged in the cavitylength direction at a pitch of 203 nm. Each InAsP absorption layer 3 andthe InGaP refractive index compensation layer 18 formed thereon have atriangular cross section parallel to the cavity length direction andperpendicular to the main surfaces of the n-type InP substrate 1, and anapex of the triangle projects into the n-type InP substrate 1. Theaverage refractive index of the InAsP absorption layers 3 and the InGaPrefractive index compensation layers 18 is equal to the refractive indexof InP.

The MQW active layer 6 includes ten quantum wells, each quantum wellincluding an InGaAsP well layer (thickness: 6 nm) and an InGaAsP barrierlayer (thickness: 10 nm; λg=1.05 μm). Strain is induced by compressioninto the InGaAsP well layer, and is not induced intentionally into theInGaAsP barrier layer.

The photoluminescence wavelength of the InAsP absorption layer 3periodically formed in the cavity length direction is set to be 1.4 μm,and the oscillation wavelength from the MQW active layer 6 is set to be1.3 μm. Due to such a structure, the InAsP absorption layer 3 causes aperiodical change in the gain in the cavity length direction. Further,since the average refractive index of the InAsP absorption layer 3 andthe InGaP refractive index compensation layers 18 is equal to therefractive index of InP owing to the provision of the InGaP refractiveindex compensation layers 18, the refractive index does not changeperiodically. Only the gain changes periodically. Thus, for theabove-described reasons, laser oscillation having a single wavelength isprovided at a higher possibility than the laser oscillation generated bythe periodical change both in the refractive index and the gain.

The n-type InP cladding layer 4 can be replaced with an n-type InGaAsPoptical waveguide layer to be integral with the p-type InGaAsP opticalwaveguide layer 7.

A method for producing the DFB laser 300 will be described withreference to FIGS. 12A through 12F.

As is shown in FIG. 12A, corrugations 2 having a pitch of 203 nm and amaximum depth of approximately 100 nm are formed at the top surface ofthe n-type InP substrate 1 by holographic exposure.

Next, a hydrogen atmosphere is mixed with 100% phosphine (PH₃) at a rate100 cc/min. and 10% arsine (AsH₃) at a rate of 10 cc/min. In theresultant atmosphere, the n-type InP substrate 1 is heated at 600° C. Asa result, as is shown in FIG. 12B, the InAsP absorption layers 3 havinga thickness of approximately 50 nm are formed in the grooves of thecorrugations 2. Then, as is shown in FIG. 12C, the InGaP refractiveindex compensation layers 18 are grown thereon by MOVPE to form adiffraction grating. Thereafter, as is shown in FIG. 12D, the n-type InPcladding layer 4, the n-type InGaAsP optical waveguide layer 5, the MQWactive layer 6 having the above-described structure, and the p-typeInGaAsP optical waveguide layer 7, the p-type InP cladding layer 15 anda p-type InGaAsP capping layer 16 (λg=1.3 μm) are sequentially grown onthe InGaP refractive index compensation layers 18.

Then, as is shown in FIG. 12E, the mesa stripe is formed by etching. Thep-type InP current blocking layer 8, the n-type InP current blockinglayer 9, the p-type InP burying layer 10, and the p-type InGaAsP contactlayer 11 (λg=1.3 μm) are sequentially grown by liquid phase epitaxy. Theinsulation layer 12 formed of SiO₂ is deposited on the p-type InGaAsPcontact layer 11 and a stripe-shaped window is formed. Then, the p-typeelectrode 13 is formed by evaporation. On the bottom surface of then-type InP substrate 1, the n-type electrode 14 is formed byevaporation. The resultant body is cleaved to obtain the DFB laser 300as is shown in FIG. 12F.

EXAMPLE 4

FIG. 13 is a partially cut isometric view of a DFB laser 400 in a fourthexample according to the present invention. Identical elements as thosein the first example will bear identical reference numerals therewithand a description thereof will be omitted.

The DFB laser 400 has a mesa structure on a top main surface of ann-type InP substrate 1, the mesa structure including an n-type InPcladding layer 4 (thickness: 200 nm), an n-type InGaAsP opticalwaveguide layer 5 (thickness: 50 nm; λg=1.05 μm), an MQW active layer 6,a p-type InGaAsP optical waveguide layer 7 (thickness: 30 nm; λg=1.05μm), a first p-type InP cladding layer 15 (average thickness: 100 nm),and a second p-type InP cladding layer 17 (average thickness: 300 nm).The mesa structure is provided on the top surface of the n-type InPsubstrate 1. Laterally, the mesa structure is interposed by a p-type InPcurrent blocking layer 8 and an n-type InP current blocking layer 9formed on the n-type InP substrate 1. On the above-mentionedsemiconductor layers, a p-type InP burying layer 10 and a p-type InGaAsPcontact layer 11 (λg=1.3 μm) are formed.

On a main surface of the n-type InP substrate 1 which does not have themesa structure thereon (bottom surface), an n-type electrode 14 formedof an Au/Sn alloy is provided. On a top surface of the p-type InGaAsPcontact layer 11, an insulation layer 12 formed of SiO₂ having astripe-shaped window is formed. A p-type electrode 13 provided tosubstantially cover the insulation layer 12 is in contact with thep-type InGaAsP contact layer 11 through the window.

Between the first p-type InP cladding layer 15 and the second p-type InPcladding layer 17, a plurality of InAsP absorption layers 3 and aplurality of InGaP refractive index compensation layers 18 are arrangedin the cavity length direction at a pitch of 203 nm. Each InAsPabsorption layer 3 and the InGaP refractive index compensation layer 18formed thereon have a triangular cross section parallel to the cavitylength direction and perpendicular to the main surfaces of the n-typeInP substrate 1, and an apex of the triangle projects into the n-typeInP substrate 1.

The MQW active layer 6 includes ten quantum wells, each quantum wellincluding an InGaAsP well layer (thickness: 6 nm) and an InGaAsP barrierlayer (thickness: 10 nm; λg=1.05 μm). Strain is induced by compressioninto the InGaAsP well layer, and is not induced intentionally into theInGaAsP barrier layer.

The photoluminescence wavelength of the InAsP absorption layer 3periodically formed in the cavity length direction is set to be 1.4 μm,and the oscillation wavelength from the MQW active layer 6 is set to be1.3 μm. Due to such a structure, the InAsP absorption layer 3 causes aperiodical change in the gain in the cavity length direction. Further,since the average refractive index of the InAsP absorption layer 3 andthe InGaP refractive index compensation layers 18 is equal to therefractive index of InP owing to the provision of the InGaP refractiveindex compensation layers 18, the refractive index does not changeperiodically. Only the gain changes periodically. Thus, for theabove-described reasons, laser oscillation having a single wavelength isprovided at a higher possibility than the laser oscillation generated bythe periodical change both in the refractive index and the gain.

In this example, the InAsP absorption layers 3 are formed above the MQWactive layer 6. In the structure of the third example, the n-type InPcladding layer 4 and the n-type InGaAsP optical waveguide layer 5interposed between the InAsP absorption layers 3 and the MQW activelayer 6 each preferably have a relatively large thickness in order torecover the crystallinity. In the structure in the fourth example, thethickness of each of the p-type InGaAsP optical waveguide layer 7 andthe first p-type InP cladding layer 15 interposed between the MQW activelayer 6 and the InAsP absorption layers 3 can be set relatively freely.This indicates that the optical intensity distribution and the degree ofcoupling due to the InAsP absorption layer 3 can be set more freely thanin the third example.

The first p-type InP cladding layer 15 can be replaced with a p-typeInGaAsP optical waveguide layer to be integral with the p-type InGaAsPoptical waveguide layer 7.

A method for producing the DFB laser 400 will be described withreference to FIGS. 14A through 14F.

As is shown in FIG. 14A, the n-type InP cladding layer 4, the n-typeInGaAsP optical waveguide layer 5, the MQW active layer 6 having theabove-described structure, and the p-type InGaAsP optical waveguidelayer 7, the first p-type InP cladding layer 15 are sequentially grownon the n-type InP substrate 1 by MOVPE. Then, corrugations 2 are formedhaving a pitch of 203 nm and a maximum depth of approximately 100 nm areformed by holographic exposure.

Next, a hydrogen atmosphere is mixed with 100% phosphine (PH₃) at a rate100 cc/min. and 10% arsine (AsH₃) at a rate of 10 cc/min. In theresultant atmosphere, the n-type InP substrate 1 is heated at 600° C. Asa result, as is shown in FIG. 14B, the InAsP absorption layers 3 havinga thickness of approximately 50 nm are formed in the grooves of thecorrugations 2. Then, as is shown in FIG. 14C, the InGaP refractiveindex compensation layers 18 are grown thereon by MOVPE to form adiffraction grating. As is shown in FIG. 14D, the second p-type InPcladding layer 17 and a p-type InGaAsP capping layer 16 (λg=1.3 μm) aresequentially formed on InGaP refractive index compensation layers 18.

Thereafter, as is shown in FIG. 14E, the mesa stripe is formed byetching. Then, the p-type InP current blocking layer 8, the n-type InPcurrent blocking layer 9, the p-type InP burying layer 10, and thep-type InGaAsP contact layer 11 (λg=1.3 μm) are sequentially grown byliquid phase epitaxy. The insulation layer 12 formed of SiO₂ isdeposited on the p-type InGaAsP contact layer 11 and a stripe-shapedwindow is formed. Then, the p-type electrode 13 is formed byevaporation. On the bottom surface of the n-type InP substrate 1, then-type electrode 14 is formed by evaporation. The resultant body iscleaved to obtain the DFB laser 400 as is shown in FIG. 14F.

The DFB lasers described in the first through fourth examples generatelaser oscillation at 1.3 μm and the vicinity thereof. The presentinvention is applicable to DFB lasers designed for other wavelengthranges, such as 1.55 μm and the vicinity thereof.

The DFB lasers described in the first through fourth examples have aburied structure. The same effects can be obtained in a DFB laser havinga ridge structure.

In the first through fourth examples, the buried structure is formed byliquid phase epitaxy, but it can also be formed by MOVPE.

EXAMPLE 5

FIG. 15 is a partially cut isometric view of a DFB laser 500 in a thirdexample according to the present invention. Identical elements as thosein the first example will bear identical reference numerals therewithand a description thereof will be omitted.

The DFB laser 500 has a mesa structure on a top main surface of ann-type InP substrate 1, the mesa structure including an n-type InPcladding layer 4 (thickness: 200 nm), an n-type InGaAsP opticalwaveguide layer 5 (thickness: 50 nm; λg=1.05 μm), an MQW active layer 6,a p-type InGaAsP optical waveguide layer 7 (thickness: 30 nm; λg=1.05μm), and a p-type InP cladding layer 15 (thickness: 400 nm). The mesastructure is provided on corrugations 2 formed at the top surface of then-type InP substrate 1. Laterally, the mesa structure is interposed by ap-type InP current blocking layer 8 and an n-type InP current blockinglayer 9 formed on the n-type InP substrate 1. On the above-mentionedsemiconductor layers, a p-type InP burying layer 10 and a p-type InGaAsPcontact layer 11 (λg=1.3 μm) are formed.

On a main surface of the n-type InP substrate 1 which does not have themesa structure thereon (bottom surface), an n-type electrode 14 formedof an Au/Sn alloy is provided. On a top surface of the p-type InGaAsPcontact layer 11, an insulation layer 12 formed of SiO₂ having astripe-shaped window is formed. A p-type electrode 13 provided tosubstantially cover the insulation layer 12 is in contact with thep-type InGaAsP contact layer 11 through the window.

The InAsP absorption layers 3 are each interposed between n-type InGaAsPrefractive index compensation layers 32 (thickness: 50 nm; λg: 1.2 μm).

The MQW active layer 6 includes ten quantum wells, each quantum wellincluding an InGaAsP well layer (thickness: 6 nm) and an InGaAsP barrierlayer (thickness: 10 nm; λg=1.05 μm). Strain is induced by compressioninto the InGaAsP well layer, and is not induced intentionally into theInGaAsP barrier layer.

The photoluminescence wavelength of the InAsP absorption layer 3periodically formed in the cavity length direction is set to be 1.4 μm,and the oscillation wavelength from the MQW active layer 6 is set to be1.3 μm. Due to such a structure, the InAsP absorption layer 3 causes aperiodical change in the gain in the cavity length direction. Further,the n-type InGaAsP refractive index compensation layers 32 interposingthe InAsP absorption layers 3 significantly reduce the periodical changein the refractive index. Accordingly, substantially only the gainchanges periodically. Thus, for the above-described reasons, laseroscillation having a single wavelength is provided at a higherpossibility than the laser oscillation generated by the periodicalchange both in the refractive index and the gain.

The n-type InP cladding layer 4 can be replaced with an n-type InGaAsPoptical waveguide layer to be integral with the p-type InGaAsP opticalwaveguide layer 7.

The InAsP absorption layers 3 and the n-type InGaAsP refractive indexcompensation layers 32 can be formed in the p-type InP cladding layer15.

A method for producing the DFB laser 500 will be described withreference to FIGS. 16A through 16F and 17A through 17D.

As is shown in FIGS. 16A and 17A, an n-type InGaAsP layer and n-type InPlayer (thickness: 20 nm) are sequentially grown on the n-type InPsubstrate 1 by MOVPE. As is shown in FIGS. 16B and 17B, corrugations 2having a pitch of 203 nm and a maximum depth of approximately 100 nm areformed in the layers to form the n-type InP layer 33 and the n-typeInGaAsP refractive index compensation layers 32 by holographic exposure.Thus, a diffraction grating is formed.

Next, a hydrogen atmosphere is mixed with 100% phosphine (PH₃) at a rate100 cc/min. and 10% arsine (AsH₃) at a rate of 10 cc/min. In theresultant atmosphere, the n-type InP substrate 1 is heated at 600° C. Asa result, as is shown in FIGS. 16C and 17C, the InAsP absorption layers3 having a thickness of approximately 50 nm are formed in the grooves ofthe corrugations 2 to form a diffraction grating. Then, as is shown inFIG. 17D, the n-type InP cladding layer 4 is grown on the corrugations2. As is shown in FIG. 16D, the n-type InGaAsP optical waveguide layer5, the MQW active layer 6 having the above-described structure, and thep-type InGaAsP optical waveguide layer 7, the p-type InP cladding layer15 and a p-type InGaAsP capping layer 16 (λg=1.3 μm) are sequentiallygrown on the n-type InP cladding layer 4.

Then, as is shown in FIG. 16E, the mesa stripe is formed by etching. Thep-type InP current blocking layer 8, the n-type InP current blockinglayer 9, the p-type InP burying layer 10, and the p-type InGaAsP contactlayer 11 (λg=1.3 μm) are sequentially grown by liquid phase epitaxy. Theinsulation layer 12 formed of SiO₂ is deposited on the p-type InGaAsPcontact layer 11 and a stripe-shaped window is formed. Then, the p-typeelectrode 13 is formed by evaporation. On the bottom surface of then-type InP substrate 1, the n-type electrode 14 is formed byevaporation. The resultant body is cleaved to obtain the DFB laser 500as is shown in FIG. 16F.

EXAMPLE 6

A DFB laser having an integrated modulator is expected as a light sourcegenerating an excessively low level of wavelength chirp. With theconventional structure, the production yield is not sufficiently high.In the case of gain coupled type DFB lasers, the production yield interms of laser oscillation having a single wavelength is satisfactory,but the level of wavelength chirp cannot be sufficiently low.

A gain coupled DFB laser in a sixth example includes absorption layerswhich are formed periodically without performing etching or re-growth.

FIG. 20 is a cross sectional view along the cavity length direction of aDFB laser 600 in the sixth example according to the present invention.

The DFB laser 600 includes an emission part 71 for emitting laser lightand a modulation part 72 for modulating the laser light. The emissionpart 71 includes components integrated on an n-type InP substrate 51.The emission part 71 and the modulation part 72 are integrated in amultiple layer structure having a width of approximately 1 toapproximately 2 μm.

In more detail, the emission part 71 includes a plurality of n-typeInGaAs absorption layers 52 formed on a surface of the n-type InPsubstrate 51 to form a periodical absorption type diffraction grating(pitch: 243 nm; λg=1.68 μm; thickness: 30 nm); and an n-type InGaAsPoptical waveguide layer 55 (cladding layer) (λg=1.05 μm; thickness: 150nm), an undoped InGaAsP MQW active layer 56, a p-type InP cladding layer58A, a p-type InGaAsP contact layer 59, and a p-type electrode 60 whichare laminated on the n-type InP substrate 51 covering the n-type InGaAsabsorption layers 52.

The modulation part 72 includes an undoped InGaAsP light modulationlayer 57 (λg=1.48 μm; thickness: 300 nm), a p-type InP cladding layer58B, and a p-type electrode 61 which are laminated on the n-type InPsubstrate 51 in this order.

The p-type electrodes 60 and 61 are electrically separated from eachother to be supplied with different potentials. An n-type electrode 62is formed entirely on a bottom surface of the n-type InP substrate 51and acts as a common n-type electrode for the emission part 71 and themodulation part 72. Across the p-type electrode 60 and the n-typeelectrode 62, a constant voltage is supplied by a voltage applicationdevice (not shown) to cause an electric current through the emissionpart 71. As a result, stable laser oscillation is generated. The p-typeelectrode 61 is supplied with a modulated voltage in a reverse biasstate, so that the optical characteristics of the undoped lightmodulation layer 57 are changed in accordance with the applied voltage.

An end surface of the DFB laser 600 on the side of the modulation part72, namely, an end surface from which the laser light is to be emitted(front end), is coated with a low reflection film 64 having areflectivity of 0.1%. The other end surface of the DFB laser 600,namely, the end surface on the side of the emission part 71 (back end)is coated with a high reflection film 63 having a reflectivity of 90%.Due to such a structure, a large optical output is obtained from thefront end of the DFB laser 600.

The MQW active layer 56 includes a plurality of InGaAsP strain welllayers (thickness: 6 nm; compression strain: 1%) and a plurality ofbarrier layers (thickness: 10 nm; λg=1.3 μm) laminated alternately. Inthis example, seven quantum wells are provided. As a result of measuringthe photoluminescence from the MQW active layer 56, the effectivebandgap wavelength (λ_(PL)) of the MQW active layer 56 was found to be1.56 μm.

The n-type InGaAs absorption type diffraction grating includes aplurality of light absorption layers 52 (thickness: 30 nm) regularlyarranged in the cavity length direction (optical axis direction). Thelight absorption layers 52 each have a width (W) of 50 nm and arearranged at a pitch (A) of 243 nm. In such a structure, the absorptioncoefficient (namely, gain) of the light generated from the MQW activelayer 56 changes periodically along the cavity length direction. Thus, again coupled cavity is formed. Due to such a cavity, laser oscillationhaving a single wavelength at the Bragg wavelength (1.55 μm) or thevicinity thereof when the current is injected to the emission part 71.

The modulation part 72 modulates the laser light generated by theemission part 71 by the electric field absorption effect caused byapplication of a voltage in the reverse bias state. In this example, theoscillation threshold current in the emission part 71 was 20 mA, and theoptical output from the front end in the modulation part 72 in the stateof being supplied with no bias voltage was 10 mW. When a reverse biasvoltage of 2 V was applied, an extinction ratio of 20 dB was obtained.

A method for producing the DFB laser 600 will be described, hereinafter.

On a top surface of the n-type InP substrate 51, an n-type InGaAs filmis grown by a first step of MOVPE. The n-type InGaAs film is treated byholographic exposure and etching to form the n-type InGaAs absorptionlayers 52 to form a diffraction grating having a pitch of 243 nm only inthe area corresponding to the emission part 71.

Next, the n-type InGaAsP optical waveguide layer 55, the undoped InGaAsPMQW active layer 56, the p-type InP cladding layer 58A, and the p-typeInGaAsP contact layer 59 are grown on the entire top surface of then-type InP substrate 51, covering the n-type InGaAs absorption typediffraction grating. In the area corresponding to the modulation part72, the semiconductor layers thus grown are removed. In detail, an areaof a top surface of the p-type InGaAsP contact layer 59 corresponding tothe emission part 71 is masked by SiO₂, and the resultant lamination isetched by a mixture solution of H₂ SO₄ :H₂ O₂ :H₂ O=5:1:1 to remove apart of the p-type InGaAsP contact layer 59 corresponding to themodulation part 72. Then, the p-type InP cladding layer 58A is etched bya mixture of HCl:H₃ PO₃ =1:2, the undoped InGaAsP MQW active layer 56and the n-type InGaAsP optical waveguide layer 55 are etched by H₂ SO₄:H₂ O₂ :H₂ O=5:1:1, to remove a part of the respective layerscorresponding to the modulation part 72. On the area of the n-type InPsubstrate 51 exposed by such etching, the undoped InGaAsP lightmodulation layer 57 and the p-type InP cladding layer 58B are grown by athird step of MOVPE. Thus, the modulation part 72 is formed.

Then, in order to confine a current and light in the lateral direction,the above-formed semiconductor layers are patterned to be a stripe. Indetail, after an SiO₂ stripe mask pattern having a width ofapproximately 1 to 2 μm and extending in the cavity length direction isformed, the p-type InP cladding layer 58A and p-type InP cladding layer58B are masked by the SiO₂ stripe mask pattern, and the exposed partsthereof are removed. Thereafter, another cladding layer (not shown) isformed both in the emission part 71 and the modulation part 72, and anSiO₂ passivation film (not shown) is formed on the entire surface of theresultant lamination. Contact windows are formed in the SiO₂ passivationfilm in one prescribed area in the emission part 71 and one prescribedarea in the modulation part 72. The p-type electrodes 60 and 61 areformed in the respective contact windows, and the n-type electrode 62 isformed on the bottom surface of the n-type InP substrate 51.

Hereinafter, characteristics of the DFB laser 600 will be described.

In the conventional DFB laser having an emission part of the refractiveindex coupled type, production yield in terms of laser oscillationhaving a single wavelength is only approximately 30%. Such a lowproduction yield is caused by processing fluctuations in the phase atthe both ends, which is inevitable in this type of laser, in addition toprocessing fluctuations in the diffraction grating. In the DFB laser 600of the gain coupled type, laser oscillation having a single wavelengthis generated at a high possibility to realize the production yield of60% or more.

In the DFB laser 600, the hole burning in the axis direction and thephase fluctuation caused by the change in the refractive index aresmaller than in the conventional DFB laser of the refractive indexcoupled type. For these reasons, the change in the oscillationwavelength caused by the light returning from the front end is smaller.In the case of the conventional DFB laser of the refractive indexcoupled type, the yield of obtaining the wavelength chirp of 0.02 nm orless when converted into a digital signal is only approximately 10% evenwhen the reflectivity of the front end is as low as 0.1% or less. In theDFB laser 600, the yield of obtaining the above-mentioned wavelengthchirp is twice as high when the reflectivity of the front end is 0.1% orless.

Even in the case when the reflectivity of the front end is 0.2%, such ayield of the DFB laser 600 is approximately the same as the yield of theconventional DFB laser obtained when the reflectivity is only 0.1%. Thisindicates that less strictness is allowed for controlling the thicknessof the reflection film in the production of the DFB laser, whichfacilitates the production. Such freedom in the production makespossible to form a window structure in the vicinity of the front end,and also to provide a semi-insulation layer between the emission part 71and the modulation part 72 for increasing the resistance between theemission part 71 and the modulation part 72. Needless to say, theseextra steps improve the characteristics of the DFB laser 600.

In the DFB laser 600, the active layer and the light modulation layercan be a bulk layer or a MQW layer. The structure of the MQW layer andthe light modulation layer can be designed independently from the otherlayers. In modulation into an analog signal, the spectral line width andthe level of wavelength chirp are related to an increase of the noiseand modulation strain caused by the multiple reflection on the lighttransmission path. In order to restrict such noise and strain, a lowlevel of wavelength chirp and a large spectral line width are effective.In general direct modulation of the laser light, the level of wavelengthchirp and the spectral line width are both related with a line widthenhancement factor. In other words, as the wavelength chirp is reduced,the spectral line width is also reduced. In the structure of the DFBlaser 600, an excessively low level of chirp can be obtained with alarge spectral line width by use of a bulk active layer. Such a DFBlaser can be used in a wider range of transmission paths.

In the DFB laser 600, the coupling efficiency between the emission part71 and the modulation part 72 is approximately 90%. Such a high couplingefficiency is realized because the light modulation layer 57 and theactive layer 56 are formed on the same optical axis at a high level ofprecision by use of selective etching.

The absorption type diffraction grating 52 is formed below the activelayer 56 in the semiconductor laser device shown in FIG. 20 but can beformed above the active layer 56 with the same effects.

The cladding layer is used to confine the current and light in thisexample. A buried structure can also be used, which is formed by anotherstep of crystal growth.

EXAMPLE 7

FIG. 21 is a cross sectional view along the cavity length direction of aDFB laser 700 in a seventh example according to the present invention.Identical elements as those in the sixth example will bear identicalreference numerals therewith and a description thereof will be omitted.

The DFB laser 700 includes an emission part 71 for emitting laser lightand a modulation part 72 for modulating the laser light. The emissionpart 71 includes components integrated on an n-type InP substrate 51.The emission part 71 and the modulation part 72 are integrated in astriped multiple layer structure having a width of approximately 1 toapproximately 2 μm. The emission part 71 and the modulation part 72 havesubstantially the same structure except for the thicknesses thereof.

In more detail, the emission part 71 includes a plurality of n-typeInGaAs light absorption layers 52 formed on a surface of the n-type InPsubstrate 51 to form a periodical absorption type diffraction grating(pitch: 243 nm; λg=1.68 μm; thickness: 10 nm); and an n-type InGaAsPoptical waveguide layer 55 (cladding layer) (λg=1.05 μm; thickness: 150nm), an undoped InGaAsP MQW active layer 56, a p-type InP cladding layer58, a p-type InGaAsP contact layer 59, and a p-type electrode 60 whichare laminated on the n-type InP substrate 51 covering the n-type InGaAslight absorption layers 52.

The modulation part 72 includes an n-type InGaAsP optical waveguidelayer 55 (λg=1.05 μm; thickness: 75 nm), an-undoped InGaAsP lightmodulation layer 57, a p-type InP cladding layer 58, and a p-typeelectrode 61 which are laminated on the n-type InP substrate 51 in thisorder.

In this example, the n-type InGaAs light absorption layers 52 are formedin the modulation part 72 as well as in the emission part 71. However,the n-type InGaAs absorption layers 52 in the modulation part 72 eachhave a thickness of only approximately 5 nm or less and thus does notsubstantially function as a light absorption layer for the reasondescribed later.

The undoped InGaAsP MQW active layer 56 in the modulation part 72includes a first part of a multiple layer quantum well formed in thestriped multiple layer structure, and the undoped InGaAsP lightmodulation layer 57 includes a second part of the multiple layer quantumwell. The multiple layer quantum well includes a plurality of InGaAsPwell layers (λg as a bulk=1.62 μm) and a plurality of InGaAsP barrierlayers (λg=1.30 μm) laminated alternately (so as to form ten pairs). Themultiple layer quantum well has a thickness of 180 nm in the emissionpart 71 and 90 nm in the modulation part 72. In a transition partbetween the emission part 71 and the modulation part 72, the thicknessof the multiple layer quantum well gradually changes from 180 nm to 90nm.

The p-type electrodes 60 and 61 are electrically separated from eachother to be supplied with different potentials. An n-type electrode 62is formed entirely on a bottom surface of the n-type InP substrate 51and acts as a common n-type electrode for the emission part 71 and themodulation part 72. The p-type electrode 60 and the n-type electrode 62are supplied with substantially equal voltages by a voltage applicationdevice (not shown) to cause an electric current through the emissionpart 71. As a result, stable laser oscillation is generated. The p-typeelectrode 61 is supplied with a modulated voltage in a reverse biasstate, so that the optical characteristics of the undoped lightmodulation layer 57 are changed in accordance with the applied voltage.

An end surface of the DFB laser 600 on the side of the modulation part72, namely, an end surface from which the laser light is to be emitted(front end), is coated with a low reflection film 64 having areflectivity of 0.1%. The other end surface of the DFB laser 700,namely, the end surface on the side of the emission part 71 (back end)is coated with a high reflection film 63 having a reflectivity of 90%.Due to such a structure, a large optical output is obtained from theemission end of the DFB laser 700.

The absorption type diffraction grating includes a plurality of n-typeInGaAs light absorption layers 52 (thickness: 10 nm) regularly arrangedin the cavity length direction (optical axis direction). The n-typeInGaAs light absorption layers 52 each have a width (W) of 50 nm and arearranged at a pitch (A) of 243 nm. In such a structure, the absorptioncoefficient (namely, gain) of the light generated from the MQW activelayer 56 changes periodically along the cavity length direction. Thus, again coupled cavity is formed. Due to such a cavity, laser oscillationhaving a single wavelength (1.55 μm) at the Bragg wavelength or thevicinity thereof is obtained when the current is injected to theemission part 71.

The modulation part 72 modulates the laser light generated by theemission part 71 by the electric field absorption effect caused byapplication of a voltage in the reverse bias state. In this example, theoscillation threshold current in the emission part 71 was 20 mA, and theoptical output from the outgoing end in the modulation part 72 in thestate of being supplied with no bias voltage was 10 mW. When a reversebias voltage of 2 V was applied, an extinction ratio of 20 dB wasobtained.

In the DFB laser 700, the n-type InGaAs light absorption layers 52, theMQW active layer 56 and the undoped InGaAsP light modulation layer 57are thicker in the emission part 71 than in the modulation part 72. Inthis example, the thickness of each of the layers 52, 56 and 57 in themodulation part 72 is half the thickness thereof in the emission part71. As a result, the quantum shift quantity in each of the layers 52, 56and 57 depends on the thickness thereof. The effective bandgapwavelength of the MQW active layer 56 in the emission part 71 is 1.56 μm(well thickness: 8 nm), and the effective bandgap wavelength of thelight modulation layer 57 in the modulation part 72 is 1.49 μm (wellthickness: 4 nm). The effective bandgap wavelength of the n-type InGaAslight absorption layers 52 is 1.58 μm (well thickness: 10 nm) in theemission part 71 and is 1.48 μm (well thickness: 4 nm) in the modulationpart 72. The n-type InGaAs light absorption layers 52 have a largewavelength shift quantity because of the quantum well structure thereofsandwiched by the n-type InP substrate 51 and the n-type InGaAsP opticalwaveguide layer 55 (λg=1.05 μm) both having a high barrier.

The n-type InGaAs light absorption layer 52 has a width of 50 nm in thecavity length direction and is arranged at a pitch of 243 nm. The gaincoupled is achieved by the periodical change in the absorptioncoefficient, namely, the periodical change in the gain in the cavitylength direction. Accordingly, laser oscillation having a singlewavelength at the Bragg wavelength or the vicinity thereof is obtainedwhen the current is injected to the emission part 71.

When a forward bias voltage is applied to the emission part 71 to injectan electric current, laser oscillation having a single wavelength of1.55 μm is generated by gain coupling. The laser light which hasachieved the modulation part 72 is modulated by application of a reversebias voltage. Since the emission part 71 and the modulation part 72 areformed of layers grown in the same crystal growth step and thus arecontinuous, the optical coupling ratio is close to 100%. Further, sincethe undoped InGaAsP light modulation layer 57 and the n-type InGaAslight absorption layers 52 in the modulation part 72 have a sufficientlyhigher bandgap energy level than the energy level of the laser lightalthough these layers are formed in the same step with those in theemission part 71, the laser light is propagated through the modulationpart 72 with little loss. Due to such a low level of optical loss, whena driving current of 100 mA is supplied to the emission part 71, anoptical output as high as 10 mW or more is obtained from the outgoingend of the modulation part 72. The level of wavelength chirp is as lowas in the sixth example. Although the modulation part 72 includes theperiodical absorption type diffraction grating in this example, theeffective refractive index in the modulation part 72 is smaller than theeffective refractive index in the emission part 71. Thus, the Braggwavelength in the modulation part 72 is sufficiently shorter than thewavelength of the laser light. Accordingly, the laser light is scarcelydiffracted by the absorption type diffraction grating.

A method for producing the DFB laser 700 will be described, hereinafter.

A top surface of the n-type InP substrate 51 is covered by an SiO₂ maskhaving two stripes extending in the cavity length direction. An exampleof the mask is shown in FIG. 25B. The two stripes of the SiO₂ mask arearranged with an opening therebetween having a width of 10 μm, and thewidth of each stripe of the SiO₂ mask is set to be, for example, 30 μmin the area corresponding to the emission part 71, and set to be 10 μmin the area corresponding to the modulation part 72. Due to thedifference in the width, the width of exposed areas (openings) in theemission part 71 is different from the width of openings in themodulation part 72. Accordingly, the crystal growth rate after this stepis different in the parts 71 and 72. The crystal growth rate is lower inthe part having larger openings (the modulation part 72) than the otherpart (the emission part 71). The shape of the mask for causing suchdifferent crystal growth rates is not limited to the one shown in FIG.25B. The mask can also be formed of a specific semiconductor materialother than SiO₂ or a non-amorphous insulation material such as siliconnitride.

Next, an n-type InGaAs film is grown on the openings by a first step ofMOVPE. The thickness of the n-type InGaAs film depends on the width ofthe openings. In this example, the thickness is 10 nm in the areacorresponding to the emission part 71 and 5 nm in the area correspondingto the modulation part 72.

Then, the n-type InGaAs film is treated by holographic exposure andetching to form the n-type InGaAs light absorption layers 52 to form adiffraction grating having a pitch of 243 nm.

Next, in the state where the SiO₂ is left, the n-type InGaAsP opticalwaveguide layer 55, the undoped InGaAsP MQW active layer 56, the p-typeInP cladding layer 58, and the p-type InGaAsP contact layer 59 are grownselectively on prescribed areas by a second step of MOVPE. The thicknessof each layer depends on the width of the opening of the SiO₂, and thusa desirable thickness difference is achieved between the emission part71 and the modulation part 72.

Then, in order to confine a current and light in the lateral direction,another cladding layer (not shown) is formed in the same manner as inthe sixth example. An SiO₂ passivation film (not shown) is formed on theentire surface of the resultant lamination. Holes are formed in the SiO₂passivation film in one prescribed area in the emission part 71 and oneprescribed area in the modulation part 72. The p-type electrodes 60 and61 are formed in the respective holes, and the n-type electrode 62 isformed on the bottom surface of the n-type InP substrate 51.

In such a production method, the planar layout of the striped laminationis determined by only one masking step. Further, formation of thesemiconductor layers requires only two crystal growth steps, one beforeand the other after the formation of the diffraction grating.Especially, the use of a mask having openings which have differentwidths for the emission part 71 and the modulation part 72 has aremarkable advantage that the formation of the multiple layer structureis significantly facilitated by self alignment with the openings.

In this example, the thickness of each layer is controlled by the widthof the openings of the mask. Alternatively, such control can beperformed by the width of further holes formed in the mask so as tointerpose the openings. The multiple layer structure can be formed on amesa stripe having a changing width and formed on the top surface of then-type InP substrate 51.

EXAMPLE 8

FIG. 22 is a cross sectional view along the cavity length direction of aDFB laser 800 in an eighth example according to the present invention.Identical elements as those in the sixth example will bear identicalreference numerals therewith and a description thereof will be omitted.

The DFB laser 800 includes an emission part 71 for emitting laser lightand a modulation part 72 for modulating the laser light. The emissionpart 71 includes components integrated on an n-type InP substrate 51.The emission part 71 and the modulation part 72 are integrated in astriped multiple layer structure having a width of approximately 1 toapproximately 2 μm. The emission part 71 and the modulation part 72 havesubstantially the same structure except for the thicknesses thereof.

In more detail, the emission part 71 includes a plurality of n-typeInGaAs light absorption layers 52 which are formed on a corrugated topsurface of the n-type InP substrate 51 to form a periodical absorptiontype diffraction grating (pitch: 243 nm; λg=1.68 μm; thickness: 30 nm);and an n-type InGaAsP optical waveguide layer 55 (cladding layer)(λg=1.05 μm; thickness: 150 nm), an undoped InGaAsP MQW active layer 56,a p-type InP cladding layer 58, a p-type InGaAsP contact layer 59, and ap-type electrode 60 which are laminated on the n-type InGaAs lightabsorption layers 52 in this order.

The modulation part 72 includes an n-type InGaAsP optical waveguidelayer 55 (λg=1.05 μm; thickness: 70 nm), an undoped InGaAsP lightmodulation layer 57, a p-type InP cladding layer 58, and a p-typeelectrode 61 which are laminated on the n-type InGaAs light absorptionlayers 52.

The DFB laser 800 has the same structure with that of the DFB laser 700except for the n-type InGaAs absorption type diffraction grating.

A plurality of striped n-type InGaAs light absorption layers 52 areformed on the corrugated top surface of the n-type InP substrate 51 toform an absorption type diffraction grating. The n-type InGaAs lightabsorption layers 52 extend in a direction perpendicular to the cavitylength direction and formed selectively in the area corresponding to theemission part 71. Each n-type InGaAs light absorption layer 52 has amaximum thickness of 20 nm (effective λg=1.6 μm) in the groove of thecorrugations, but has a thickness of only several nanometers or less (PLwavelength>1.3 μm) on the ridge of the corrugations. The thickness ofthe n-type InGaAs light absorption layers 52 changes periodically inaccordance with the corrugations of the n-type InP substrate 51. Thus,the thickness of the n-type InGaAs light absorption layers 52 changesperiodically in accordance with the positional change in the bandgap. Inthis example, the n-type InGaAs light absorption layers 52 need not beseparated from each other, but it is sufficient as long as the thicknessthereof changes periodically in the cavity length direction. The n-typeInGaAs light absorption layers 52 can be separated from each other, inwhich case no layer is existent on the ridges of the corrugations. Inthe modulation part 72, the thickness of the n-type InGaAs lightabsorption layers 52 is 4 nm.

The MQW active layer 56 has a well thickness of 8 nm and an effectivebandgap wavelength (λg) of 1.56 μm. The undoped InGaAsP light modulationlayer 57 has a well thickness of 4 nm and an effective bandgapwavelength (λg) of 1.48 μm.

A method for producing the DFB laser 800 will be described, hereinafter.

On an area of a top surface of the n-type InP substrate 51 correspondingto the emission part 71, corrugations having a pitch of 243 nm and amaximum depth of approximately 50 to 100 nm are selectively formed byholographic exposure and etching.

On such a corrugated top surface of the n-type InP substrate 51, an SiO₂mask having a plurality of stripes extending in the cavity lengthdirection is formed. In the area corresponding to the emission part 71,each stripe has a width of 30 μm and are arranged with an openingtherebetween having a width of 10 μm. In the area corresponding to themodulation part 72, each stripe has a width of 10 μm and are arrangedwith an opening therebetween having a width of 10 μm. Each stripe iswider in the area corresponding to the emission part 71 than in the areacorresponding to the modulation part 72.

Next, the n-type InGaAs light absorption layers 52, the n-type InGaAsPoptical waveguide layer 55, the undoped InGaAsP MQW active layer 56, theundoped InGaAsP light modulation layer 57, the p-type InP cladding layer58, and the p-type InGaAsP contact layer 59 are grown on the n-type InPsubstrate 51 by MOVPE in the openings of the SiO₂ mask. The thickness ofeach layer depends on the width of the openings, and is different in theemission part 71 and the modulation part 72. Since the crystal growthrate is different in an area corresponding to the ridge of thecorrugations and an area corresponding to the groove of thecorrugations, the thickness of the n-type InGaAs light absorption layers52 changes periodically as is shown in FIG. 22.

Then, in order to confine a current and light in the lateral direction,another cladding layer (not shown) is formed in the same manner as inthe sixth example. An SiO₂ passivation film (not shown) is formed on theentire surface of the resultant lamination. Holes are formed in the SiO₂passivation film in one prescribed area in the emission part 71 and oneprescribed area in the modulation part 72. The p-type electrodes 60 and61 are formed in the respective holes, and the n-type electrode 62 isformed on the bottom surface of the n-type InP substrate 51.

In such a production method, the planar layout of the striped laminationis determined by only one masking step. Further, formation of thesemiconductor layers requires only one crystal growth step after theformation of the diffraction grating. Especially, the use of a maskhaving openings which have different widths for the emission part 71 andthe modulation part 72 has a remarkable advantage in that the formationof the multiple layer structure is significantly facilitated by selfalignment with the openings. The advantage that the formation of thesemiconductor layers requires only one continuous crystal growth stepsignificantly simplifies the formation process of a complicatedintegrated structure, since crystal growth is one of the most time andlabor consuming process steps in the production of the laser.

In the DFB laser 800 in this example, the same satisfactory effects asin the seventh example have been confirmed. Further, the DFB laser 800has another advantage that the diffraction grating can be formed byprocessing InP which is relatively easy in shape control by etching.Moreover, since the diffraction grating is formed simply by growing thelight absorption layers 52 by MOVPE instead of deposition and etching,the periodical change in the absorption coefficient can be obtained withcertainty even if the shape of the corrugations is not perfectlyuniform. Accordingly, even if wet etching is used to form thecorrugations at a surface of the n-type InP substrate 51 by wet etching,the characteristics of the DFB laser 800 are relatively stable.

EXAMPLE 9

FIG. 23 is a cross sectional view along the cavity length direction of aDFB laser 900 in a ninth example according to the present invention.Identical elements as those in the sixth example will bear identicalreference numerals therewith and a description thereof will be omitted.

The DFB laser 900 includes an emission part 71 for emitting laser lightand a modulation part 72 for modulating the laser light. The emissionpart 71 includes components integrated on an n-type InP substrate 51.The emission part 71 and the modulation part 72 are integrated in astriped multiple layer structure having a width of approximately 1 toapproximately 2 μm.

In more detail, the emission part 71 includes an undoped InGaAsP opticalwaveguide layer 55 (cladding layer) (λg=1.48 μm; thickness: 0.5 μm), anundoped InP spacer layer 81 (thickness: 0.1 μm), an undoped InGaAsPactive layer 56 (λg=1.55 μm; thickness: 0.1 μm), a p-type InP claddinglayer 58, a p-type InGaAsP contact layer 59, and a p-type electrode 60which are laminated on the n-type InP substrate 51 in this order. Thep-type InP cladding layer 58 includes a plurality of n-type InGaAs lightabsorption layers 52 forming a periodical absorption type diffractiongrating at a level away from the active layer 56 by a prescribeddistance. The light absorption layers 52 forming the n-type InGaAsabsorption type diffraction grating are periodically arranged at a pitchof 230 nm along the cavity length direction.

The modulation part 72 includes an undoped InGaAsP light modulationlayer 57 (composition wavelength: 1.48 μm), an undoped InP spacer layer81, a p-type InP cladding layer 58, and a p-type electrode 61 which arelaminated on the n-type InP substrate 51 in this order. The n-type InPsubstrate 51 has an n-type electrode 62 on a bottom surface thereof.

The outgoing end of the modulation part 72 is coated with a lowreflection film 64 having a reflectivity of 0.1%, and the emission endof the emission part 71 is coated with a high reflection film 63 havinga reflectivity of 90%. Due to such a structure, a large optical outputis obtained from the outgoing end.

In the DFB laser 900, the optical waveguide layer 55 and the lightmodulation layer 57 are formed of the same semiconductive material. Thespacer layer 81 is also formed in the same semiconductive material inthe emission part 71 and the modulation part 72.

A method for producing the DFB laser 900 will be described, hereinafter.

On a top surface of the n-type InP substrate 51, the undoped InGaAsPoptical waveguide layer 55, the undoped spacer layer 81, the undopedInGaAsP active layer 56, a p-type InP cladding layer, and an n-typeInGaAsP contact layer are grown on the n-type InP substrate 51 by MOVPE.Then, the n-type InGaAs layer is treated with holographic exposure andetching to form a plurality of the n-type InGaAs light absorption layers52 arranged at a pitch of 230 nm in the cavity length direction.

After an area of a top surface of the p-type InP cladding layer 58corresponding to the emission part 71 is covered with a SiO₂ mask, then-InGaAs contact layer in the area corresponding to the modulation part72 is selectively etched away by a mixture solution of H₂ SO₄ :H₂ O₂ :H₂O=5:1:1. Then, the p-type InP cladding layer is etched by a mixture ofHCl:H₃ PO₄ =1:2, the undoped InGaAsP MQW active layer 56 and the n-typeInGaAsP optical waveguide layer 55 are etched by H₂ SO₄ :H₂ O₂ :H₂O=5:1:1, to remove a part of the respective layers corresponding to themodulation part 72.

Then, by a second step of MOVPE, the p-type cladding layer 58 and thep-type InGaAsP contact layer 59 are grown.

Then, in order to confine a current and light in the lateral direction,an SiO₂ mask having stripes extending in the cavity pattern is formed,and the layers to the p-type InP cladding layer 58 are removed. Then,another cladding layer (not shown) is formed in each of the emissionpart 71 and the modulation part 72. An SiO₂ passivation film (not shown)is formed on the entire surface of the resultant lamination. Holes areformed in the SiO₂ passivation film in one prescribed area in theemission part 71 and one prescribed area in the modulation part 72. Thep-type electrodes 60 and 61 are formed in the respective holes, and then-type electrode 62 is formed on the bottom surface of the n-type InPsubstrate 51.

In such a production method, the planar layout of the striped laminationis determined by only one masking step. Further, formation of thesemiconductor layers requires only two crystal growth steps, one beforeand one after the formation of the diffraction grating. Moreover, anarbitrary structure can be selected for the active layer 56 and thelight modulation layer 57 within the range in which characteristics ofthe laser in the emission part 71 and the optical coupling between theemission part 71 and the modulation part 72 are not deteriorated. Forexample, the active layer 56 and the light modulation layer 57 can eachhave a bulk structure or an MQW structure. In the case of the bulkstructure is used, the wavelength chirp can be reduced without reducingthe spectral line width of the light source. Such an advantage iseffective in restricting additional strain and noise on the analog lighttransmission path having multiple reflectivity. Thus, a wide range ofthe spectral line width can be provided by a simple production process.

EXAMPLE 10

FIG. 24 is a cross sectional view along the cavity length direction of aDFB laser 1000 in a tenth example according to the present invention.Identical elements as those in the sixth example will bear identicalreference numerals therewith and a description thereof will be omitted.

The DFB laser 1000 includes an emission part 71 for emitting laser lightand a modulation part 72 for modulating the laser light. The emissionpart 71 includes components integrated on an n-type InP substrate 51.The emission part 71 and the modulation part 72 are integrated in astriped multiple layer structure having a width of approximately 1 toapproximately 2 μm. The emission part 71 and the modulation part 72 havesubstantially the same structure except for the thicknesses thereof.

In more detail, the emission part 71 includes a plurality of n-typeInAsP light absorption layers 52 formed on corrugations formed atsurface of the n-type InP substrate 51 to form a diffraction gratingperiodically arranged at a pitch of 201 nm (λg=1.4 μm), an n-typeInGaAsP optical waveguide layer 55 (cladding layer) (λg=1.05 μm;thickness: 150 nm), an undoped InGaAsP MQW active layer 56, a p-type InPcladding layer 58, a p-type InGaAsP contact layer 59, and a p-typeelectrode 60 which are laminated on the n-type InP substrate 51 in thisorder. The n-type InAsP light absorption layers 52 absorb the light fromthe MQW active layer 56.

The modulation part 72 includes an n-type InGaAsP optical waveguidelayer 55 (λg=1.05 μm; thickness: 70 nm), an undoped InGaAsP lightmodulation layer 57, a p-type InP cladding layer 58, and a p-typeelectrode 61 which are laminated on the n-type InP substrate in thisorder. The n-type InP substrate 51 has an n-type electrode 62 on abottom surface thereof.

The undoped InGaAsP MQW active layer 56 has a well thickness of 8 nm anda PL wavelength of 1.31 μm, and the undoped InGaAsP light modulationlayer 57 has a well thickness of 4 nm and a PL wavelength of 1.25 μm.

The DFB laser 1000 has the same structure with the DFB laser 80D shownin FIG. 22 except for the n-type InAsP light absorption layers 52forming the diffraction grating.

The n-type InAsP light absorption layers 52 are formed in stripes inaccordance with corrugations formed at a top surface of the n-type InPsubstrate 51. Each stripe of the n-type InAsP light absorption layers 52are extended in a direction perpendicular to the cavity length directionand selectively formed in an area corresponding to the emission part 71.The n-type InAsP light absorption layers 52 each have a maximumthickness of 20 nm (effective λg=1.6 μm) in the groove of thecorrugations but have a thickness of only several nanometers or less (PLwavelength>1.3 μm) on the ridge of the corrugations. The thickness ofthe n-type InAsP light absorption layers 52 changes periodically inaccordance with the corrugations of the n-type InP substrate 51. Thus,the thickness of the n-type InAsP light absorption layers 52 changesperiodically in accordance with the positional change in the bandgap. Inthis example, the n-type InAsP light absorption layers 52 need not beseparated from each other, but it is sufficient as long as the thicknessthereof changes periodically in the cavity length direction. The n-typeInAsP light absorption layers 52 can be separated from each other, inwhich case no layer is existent on the ridges of the corrugations. Inthe modulation part 72, the thickness of the n-type InAsP lightabsorption layers 52 is 4 nm.

A method for producing the DFB laser 1000 will be described withreference to FIGS. 25A through 25C.

As is shown in FIG. 25A, on an area of a top surface of the n-type InPsubstrate 51 corresponding to the emission part 71, corrugations havinga pitch of 230 nm and a maximum depth of approximately 50 to 100 μm areselectively formed by holographic exposure and etching to form thediffraction grating.

Next, as is shown in FIG. 25B, on such a corrugated top surface of then-type InP substrate 51, an SiO₂ mask having a pair of stripes extendingin the cavity length direction is formed. In the area corresponding tothe emission part 71, each stripe has a width of 30 μm and is arrangedwith an opening therebetween having a width of 10 μm. In the areacorresponding to the modulation part 72, each stripe has a width of 10μm and are arranged with an opening therebetween having a width of 10μm. Each stripe is wider in the area corresponding to the emission part71 than in the area corresponding to the modulation part 72.

Next, a hydrogen atmosphere is supplied with 100% phosphine (PH₃) at arate 100 cc/min. and 10% arsine (AsH₃) at a rate of 10 cc/min. In theresultant atmosphere, the n-type InP substrate 51 is heated at 600° C.As a result, as is shown in FIG. 25C, the InAsP absorption layers 52having a thickness of approximately 50 nm are formed in the grooves ofthe corrugations to form a diffraction grating. Then, the n-type InGaAsPoptical waveguide layer 55 the undoped InGaAsP MQW active layer 56, theundoped InGaAsP light modulation layer 57, the p-type InP cladding layer58, and the p-type InP contact layer 59 are grown by MOVPE in theopenings of the SiO₂ mask. The thickness of each layer depends on thewidth of the openings, and is different in the emission part 71 and themodulation part 72.

Then, in order to confine a current and light in the lateral direction,another cladding layer (not shown) is formed in the same manner as inthe sixth example. An SiO₂ passivation film (not shown) is formed on theentire surface of the resultant lamination. Holes are formed in the SiO₂passivation film in one prescribed area in the emission part 71 and oneprescribed area in the modulation part 72. The p-type electrodes 60 and61 are formed in the respective holes, and the n-type electrode 62 isformed on the bottom surface of the n-type InP substrate 51.

In such a production method, the planar layout of the striped laminationis determined by only one masking step. Further, formation of thesemiconductor layers requires only one crystal growth step after theformation of the diffraction grating. Especially, the use of a maskhaving openings which have different widths for the emission part 71 andthe modulation part 72 has a remarkable advantage that the formation ofthe multiple layer structure is significantly facilitated by selfalignment with the openings. The advantage that the formation of thesemiconductor layers requires only one continuous crystal growth stepsignificantly simplifies the formation process of a complicatedintegrated structure, since the crystal growth is one of the most timeand labor consuming process steps in the production of the laser.

EXAMPLE 11

Next, referring to FIG. 26, a distributed Bragg reflective laser (DBR)laser in the eleventh example according to the present invention will bedescribed. FIG. 26 is a cross sectional view along the cavity lengthdirection of a DBR laser in the eleventh example according to thepresent invention.

The DBR laser shown in FIG. 26 has a similar configuration to that ofthe DFB laser shown in FIG. 24. The DBR laser shown in FIG. 26 isdifferent from the DFB laser shown in FIG. 24 in that an n-type InAsPlayer 152 of this example does not form a diffraction grating for adistributed feedback, but a diffraction grating for a distributed Braggreflection (DBR). A plurality of n-type InAsP layers 152 areperiodically arranged at a pitch of 243 nm, and is surrounded by InP.The DBR diffraction grating is provided in a waveguide part 70. Thelight having a selected wavelength of the light formed by the emissionpart 71 is reflected by a reflector of the waveguide part 70 so as tooscillate at a unitary wavelength.

An undoped InGaAs MQW active layer 56 has the same structure as that ofthe active layer 56 shown in FIG. 21, and generates laser light having awavelength of 1.55 μm. In this example, the bandgap of the n-type InAsPlayers 152 is adjusted so as not to absorb the laser light.

Therefore, the plurality of n-type InAsP layers 152 form a structurewhere a refractive index periodically changes, i.e., a Bragg reflector,in the waveguide part 70. An electrode is not required to be provided onthe waveguide part 70. However, if an electrode is provided on thewaveguide part 70, a forward bias is applied and a current is supplied,then the difference in the refractive indices in the diffraction gratingis varied in accordance with the amount of the current. As a result, thewavelength of the laser light can be tuned.

EXAMPLE 12

Next, referring to FIG. 27, a DBR laser in a twelfth example accordingto the present invention will be described. FIG. 27 is a cross sectionalview along the cavity length direction of a DBR laser in the twelfthexample according to the present invention.

The DBR laser shown in FIG. 27 has a similar configuration to that ofthe DBR laser shown in FIG. 26. The DBR laser shown in FIG. 27 isdifferent from the DBR laser shown in FIG. 26 in that a phase controlelectrode 70a and a wavelength tuning electrode 70b are provided on thewaveguide part 70 in this example. The phase control electrode 70a isdisposed on a region of the waveguide part 70 where the diffractiongrating is not provided.

In increasing the forward voltage applied between the wavelength tuningelectrode 70b and the electrode 62, a sudden discontinuous change issometimes caused in the oscillation wavelength. The phase controlelectrode 70a functions for preventing such a discontinuous change ofthe oscillation wavelength. If a forward voltage is applied between thephase control electrode 70a and the electrode 62, then the discontinuouschange of the oscillation wavelength can be prevented.

The DFB lasers described in the fifth through tenth examples generatelaser oscillation at 1.3 μm and in the vicinity thereof usingInGaAsP/InP materials. The present invention is applicable to DFB lasersdesigned for other wavelength ranges, such as 1.55 μm and in thevicinity thereof.

The bandgap and thickness of each semiconductor layer can be changedwithin the scope of the present invention. The present invention isapplicable to a DFB laser formed of AlGaAs/GaAs and other materials aswell.

As has been described so far, a DFB laser according to the presentinvention generates laser oscillation having a single wavelength, andthe oscillation mode is not changed much by the light returning fromoutside or affected by noise.

In a production method of a DFB laser according to the presentinvention, periodical light absorption layers are formed withoutetching. Accordingly, the semiconductor layers formed after the lightabsorption layers have satisfactory quality and are reliable for a longperiod of time. The thickness and the positional arrangement of thelight absorption layers are controlled more precisely due to theelimination of etching in the formation thereof. The whole productionprocess is simplified and is used for various types of DFB lasers.

According to the present invention, DFB lasers which provide asufficiently low level of wavelength chirp and thus can be designed withmore flexibility in terms of the spectral line width are produced at ahigh production yield. Such a method for producing various type of lightsources having a single wavelength and an excessively low level ofwavelength chirp by a very simple process is valuable in practice.

Various other modifications will be apparent to and can be readily madeby those skilled in the art without departing from the scope and spiritof this invention. Accordingly, it is not intended that the scope of theclaims appended hereto be limited to the description as set forthherein, but rather that the claims be broadly construed.

What is claimed is:
 1. A semiconductor laser, comprising an InPsubstrate and a multiple layer structure formed on a main surface of theInP substrate, wherein:the multiple layer structure comprises at leastan active layer for emitting laser light and a periodical structure fordistributed feedback of the laser light, the periodical structurecomprises a plurality of semiconductor regions each having asubstantially triangular cross section in a direction perpendicular tothe main surface of the InP substrate and parallel to a cavity length ofthe semiconductor laser, the triangular cross section projecting towardthe InP substrate, and the semiconductor regions are set to have abandgap energy which is smaller than the energy of light emitted fromthe active layer, and the semiconductor regions are formed of InAsP,each one of the semiconductor regions having a thickness of more than 10nm.
 2. A semiconductor laser according to claim 1, wherein theperiodical structure is provided between the InP substrate and theactive layer.
 3. A semiconductor laser according to claim 1, wherein thesemiconductor regions are provided in an InP cladding layer.
 4. Asemiconductor laser according to claim 1, wherein the semiconductorregions are provided between an InP cladding layer and an InGaAsPoptical waveguide layer.
 5. A semiconductor laser according to claim 1,wherein the active layer has a quantum well structure comprising atleast a well layer and a barrier layer.
 6. A semiconductor laseraccording to claim 5, wherein a compression strain is induced into thewell layer.
 7. A semiconductor laser according to claim 6, wherein thecompression strain is induced at 0.5 to 1.5%.
 8. A semiconductor laseraccording to claim 1, wherein the semiconductor regions are formed atleast of a first semiconductor material having a bandgap energy which issmaller than the energy of light emitted from the active layer and asecond semiconductor material having a bandgap energy which is largerthan the energy of the light.
 9. A semiconductor laser according toclaim 1, wherein the semiconductor regions have an average refractiveindex which is substantially equal to the refractive index of InP.
 10. Asemiconductor laser according to claim 8, wherein the semiconductorregions are provided in an InP cladding layer.
 11. A semiconductor laseraccording to claim 8, wherein the semiconductor regions are providedbetween an InP cladding layer and an InGaAsP optical waveguide layer.12. A semiconductor laser according to claim 8, wherein the active layerhas a quantum well structure comprising at least a well layer and abarrier layer.
 13. A semiconductor laser according to claim 12, whereina compression strain is induced into the well layer.
 14. A semiconductorlaser according to claim 13, wherein the compression strain is inducedat 0.5 to 1.5%.
 15. A semiconductor laser according to claim 8, whereinthe first semiconductor material is InAsP, and the second semiconductormaterial is InGaP.
 16. A semiconductor laser according to claim 1,wherein the semiconductor regions are formed at least of a firstsemiconductor material having a bandgap energy which is smaller than theenergy of light emitted from the active layer and a second semiconductormaterial compensating for refractivity of the first semiconductormaterial.
 17. A semiconductor laser according to claim 1, wherein theperiodical structure includes periodical corrugations, and thesemiconductor regions are provided in concave portions of the periodiccorrugation so as to be separated from each other.
 18. A semiconductorlaser according to claim 17, wherein a height of the semiconductorregions is less than a height of the periodical corrugations.
 19. Asemiconductor laser, comprising an InP substrate and a multiple layerstructure formed on a main surface of the InP substrate, wherein:themultiple layer structure comprises at least an active layer for emittinglaser light and a periodical structure for distributed feedback of thelaser light, the periodical structure comprises a plurality ofsemiconductor regions each having a substantially triangular crosssection in a direction perpendicular to the main surface of the InPsubstrate and parallel to a cavity length of the semiconductor laser,the triangular cross section projecting toward the InP substrate, andthe semiconductor regions are formed of InAsP and set to have a bandgapenergy which is equal to or larger than the energy of light emitted fromthe active layer.
 20. A semiconductor laser according to claim 19,wherein the periodical structure includes periodical corrugations, andthe semiconductor regions are provided in concave portions of theperiodic corrugation so as to be separated from each other.
 21. Asemiconductor laser according to claim 20, wherein a height of thesemiconductor regions is less than a height of the periodicalcorrugations.
 22. A semiconductor laser according to claim 19, where inthe periodical structure is provided between the InP substrate and theactive layer.
 23. A semiconductor laser according to claim 19, whereinthe semiconductor regions are provided in an InP cladding layer.
 24. Asemiconductor laser according to claim 19, wherein the semiconductorregions are provided between an InP cladding layer and an InGaAsPoptical waveguide layer.
 25. A semiconductor laser according to claim19, wherein the active layer has a quantum well structure comprising atleast a well layer and a barrier layer.
 26. A semiconductor laseraccording to claim 25, wherein a compression strain is induced into thewell layer.
 27. A semiconductor laser according to claim 26, wherein thecompression strain is induced at 0.5 to 1.5%.
 28. A semiconductor laser,comprising:a striped multiple layer structure comprising a lightemission part for emitting laser light and a modulation part which isoptically coupled with the emission part for modulating the laser light;and a semiconductor substrate for supporting the striped multiple layerstructure, wherein: the emission part comprises an active layer and aplurality of semiconductor regions respectively provided in concaveportions of periodical corrugations disposed in a direction of anoptical axis so as to be separated from each other, the semiconductorregions being set to have a bandgap energy which is smaller than theenergy of light emitted from the active layer, the semiconductor regionsbeing formed of InAsP, each one of the semiconductor regions having athickness of more than 10 nm, and the modulation part comprises a lightmodulation layer having optical characteristics which change inaccordance with a modulation signal.
 29. A semiconductor laser accordingto claim 28, wherein:the striped multiple layer structure comprises amultiple quantum well layer, the active layer in the emission partcomprises a first part of the multiple quantum well layer, and the lightmodulation layer in the modulation part comprises a second part of themultiple quantum well layer.
 30. A semiconductor laser according toclaim 29, wherein the active layer in the emission part and the lightmodulation layer in the modulation part are connected to each other by athird part of the multiple quantum well layer located between the firstpart and the second part.
 31. A semiconductor laser according to claim29, wherein the first part of the multiple quantum well layer is thickerthan the second part of the multiple quantum well layer.
 32. Asemiconductor laser according to claim 28, wherein the semiconductorregions function as a plurality of light absorption layers each having asubstantially triangular cross section projecting toward the InPsubstrate so as to constitute an absorption type diffraction gratingarranged in the direction of the optical axis.
 33. A semiconductor laseraccording to claim 32, wherein the plurality of light absorption layerseach have a thickness changing periodically in the direction of theoptical axis and have a bandgap changing periodically in accordance withthe periodical change in the thickness.
 34. A semiconductor laseraccording to claim 33, wherein the light absorption layers have aquantum well structure.
 35. A semiconductor laser according to claim 28,wherein the semiconductor regions are grown on the periodicalcorrugations.
 36. A semiconductor laser according to claim 28, whereinthe semiconductor regions are provided between the active layer and thesemiconductor substrate.
 37. A semiconductor laser according to claim28, wherein the active layer is provided between the semiconductorregions and the semiconductor substrate.
 38. A semiconductor laseraccording to claim 28, further comprising an optical waveguide layerprovided between the active layer and the semiconductor regions.
 39. Asemiconductor laser according to claim 28, further comprising a firstvoltage application device for applying a substantially constant voltageto the emission part and a second voltage application device forapplying a modulation voltage to the modulation part.
 40. Asemiconductor laser, comprising:a striped multiple layer structurecomprising a light emission part for emitting laser light and an opticalwaveguide part which is optically coupled with the light emission partfor propagating the laser light therethrough; and a semiconductorsubstrate for supporting the striped multiple layer structure, wherein:the light emission part comprises an active layer radiating the laserlight, the optical waveguide part comprises a Bragg reflector where arefractive index periodically changes along an optical axis direction,and the Bragg reflector comprises an InAsP layer formed in concaveportions of periodical corrugations formed on the semiconductorsubstrate so as to reflect light having a selected wavelength of thelaser light radiated from the active layer of the light emission parttoward the active layer.
 41. A semiconductor laser according to claim40, wherein the InAsP layer has a bandgap not to absorb the laser light.42. A semiconductor laser according to claim 41, wherein the opticalwaveguide part comprises a wavelength tuning part for adjusting awavelength of the laser light.
 43. A semiconductor laser according toclaim 42, wherein the optical waveguide part comprises a phase controlpart for adjusting a phase of the laser light.
 44. A semiconductorlaser, comprising:a striped multiple layer structure, comprising a lightemission part for emitting laser light and a modulation part which isoptically coupled with the emission part for modulating the laser light;and a semiconductor substrate for supporting the striped multiple layerstructure, wherein the emission part comprises an active layer and aplurality of semiconductor regions respectively provided in concaveportions of periodical corrugations disposed in a direction of anoptical axis so as to be separated from each other, the semiconductorregions being formed of InAsP and set to have a bandgap energy which isequal to or larger than the energy of light emitted from the activelayer, and the modulation part comprises a light modulation layer havingoptical characteristics which change in accordance with a modulationsignal.
 45. A semiconductor laser comprising:a striped multiple layerstructure, comprising a light emission part for emitting laser light anda modulation part which is optically coupled with the emission part formodulating the laser light; and a semiconductor substrate for supportingthe striped multiple layer structure, wherein the emission partcomprises an active layer, the modulation part comprises a lightmodulation layer having optical characteristics which change inaccordance with a modulation signal, an absorption type diffractiongrating periodically arranged in the optical axis direction, provided inthe emission part and the modulation part, and the laser light emittedfrom the active layer is not substantially absorbed at the absorptiontype diffraction grating in the modulation part.